If we don't, this can prevent reconnection since the task is still running, and the previous task instance could be hanging on a previous request that has never completed (i.e network issue).

Yes, there's no timeout on request coming from the publish_loop, so we
must ensure the publish_task is always cancelled. Hence, the
uaclient.close_session is always called on disconnect.

Upgrade test cases from ubuntu xenial to bionic
Update README

Mentioned in #314

String are immutable, therefore the password is never encoded and it
throws this error when it's a string:

```
...../cryptography/hazmat/backends/openssl/backend.py in _load_key(self, openssl_read_func, convert_func, data, password)
   1284         userdata = self._ffi.new("CRYPTOGRAPHY_PASSWORD_DATA *")
   1285         if password is not None:
-> 1286             utils._check_byteslike("password", password)
   1287             password_ptr = self._ffi.from_buffer(password)
   1288             userdata.password = password_ptr

...../cryptography/utils.py in _check_byteslike(name, value)
     36         memoryview(value)
     37     except TypeError:
---> 38         raise TypeError("{} must be bytes-like".format(name))
     39
     40

TypeError: password must be bytes-like
```

This patch fixes this as well as some typing inaccuracy introduced by #281

This has apparently been mistakenly removed [by this
commit](https://github.com/FreeOpcUa/opcua-asyncio/commit/234008be689901ba2f9d32606944474a61f84801#diff-0ccb06a6a30b67aafbe098c395870604R4)

And it's still required by
[security_policies](https://github.com/FreeOpcUa/opcua-asyncio/blob/master/asyncua/crypto/security_policies.py#L287)

Related issue found: https://github.com/FreeOpcUa/opcua-asyncio/issues/195

Before patch:
```
In [2]: from asyncua.crypto import uacrypto
In [3]: raise uacrypto.InvalidSignature
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-3-93aef37181e3> in <module>
----> 1 raise uacrypto.InvalidSignature

AttributeError: module 'asyncua.crypto.uacrypto' has no attribute 'InvalidSignature'
```

After:
```
In [2]: from asyncua.crypto import uacrypto
In [3]: raise uacrypto.InvalidSignature
---------------------------------------------------------------------------
InvalidSignature                          Traceback (most recent call last)
<ipython-input-13-93aef37181e3> in <module>
----> 1 raise uacrypto.InvalidSignature

InvalidSignature:

AttributeError: module 'asyncua.crypto.uacrypto' has no attribute 'InvalidSignature'
```

* Check all zero ID instead of single zero in ID.

* Type checks in __init__ to avoid problems with has_null_identifier()

* Added str as valid ByteString and expecting an exception on invalid NodeId creation

* More generic type check and better exception text

* Move type check into separate method

* Rename identifier type check method

* return instead of break
Co-authored-by: Randolph Busch <randolph.busch@ticos.ch>

OutputArguments can possibly come back as a null array, in which case asyncua unpacks as None.

This commits adds a server fixture running in a dedicated thread to handle OPC-UA
requests properly (the existing server fixture accept TCP connections
but are stuck at hello).

We then use this fixture for the CLI tests.

Also fixing the uaserver cli, its event_loop was stuck in a not async
sleep preventing it from handling requests.

recreated after branch deleted: because of #286

make the detailed policy ID match the signature algorithm and its other implications.

* implement oroulet changes

* closing_task must be a task not onl run once

* do not catch CancelleError in closing_Task

* make sure we use the correct loop in asyncio binary_server
Co-authored-by: Joey Faulkner <joeymfaulkner@gmail.com>

accidentally wrote PostWrite instead of PreWrite

* getting started

* remove ridiculous change to generated code

* remove ridiculous change to generated code

* checkpoint

* another checkpoint

* found the error, tests now pass

* improve error when person with incorrect credentials tries to connect

* add better error handling for when a user does something they aren't permitted to do

* pass tests

* add tests for simple permissions

* add third certificate and some documentation

* clean up pull request

* remove pointless import

* hopefully pass tests

* add write subscribe option and tests

* implement changes requested

* add dataclasses to requirements for python 3.6

* put dataclasses in the travis file instead of the dev_requirements.txt

* implement user manager in iserver instead of using the certificate handler

* uncomment some code

* add permissions=None to base security policy object

* pass crypto connect tests

* fix tests

* hopefully pass all the tests now

* actually pass tests this time

* add user to write callback

* allow callbacks to be asynchronous

* remove certificate handler and deal with permissions exclusively with CertificateUserManager

* remove commented out code

* add ability to find endpoints for unauthenticated user

* add pytest mock to travis file

* try to make the file less complex to pass tests

* add pytest mocking to setup.py tests_require

* rename callback dispatcher to callback service

* implement 2/3 changes requested

* fix tests

* add callbacks before write and before/after read

* fix callback tests (i think)

Iterating over the references list and deleting inside the same list
resulted in dangling inverse references if nodes where deleted.

Fix this by iterating over a copy of the references list.

Was previously fixed in https://github.com/FreeOpcUa/opcua-asyncio/commit/4da3e7e827309fe5ffe17bb5f6d84ed56b0dcf6e and subsequently reverted in https://github.com/FreeOpcUa/opcua-asyncio/commit/4268d68595f02b2ae098d9f2844788ec4b4efe21.

Resolves issue #263