Fixes

In uaserver, --populate flag was inverted and uaserver didn't work
without --populate (undefined variable myvar).
InternalServer always returned only the first endpoint

Fix authentication by certificate

Support client-side encrypted passwords

Checked on OPC Foundation .net UA server

Use IntEnum where possible instead of simple class

Clean up client's security policy API

Also, move all crypto functions to uacrypto.

Added set_security() and set_security_string() methods to Client.
Removed duplicate server_certificate field, renamed
client_certificate and private_key to user_certificate and user_private_key.

pypy3 on travis

XML importer typo fix

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.