Sync with master

fix deadlock

if two thread using internal seesion to create moniter items and change data value concurently, MonitoredItemService.create_monitored_items & AddressSpace.set_attribute_value may deadlock.

PEP-8 cleanup

tests for command lines

add shadow copy to Buffer

Set default security policy

I've broken command-line tools in previous commit

Security policies implementation

Split messages into MessageChunks

Client now successfully connects to OPC foundation .net server.
Dependency on cryptography module is completely optional: you
can use python-opcua without cryptography if you only use #None policy.

Client and server should respect BufferSize values and
should not send chunks bigger than peer's BufferSize.
Also, added base class for SecurityPolicy. Only None policy
(no signing, no encryption) is implemented now.

Skip typo check if PYOPCUA_NO_TYPO_CHECK is defined in env

If define "PYOPCUA_NO_TYPE_CHECK" environment variable, typo check is ignore. It is a performance option for running opcua in production

implement filtering in server side find_servers

performance enhancement 2