Port to PVXS

[mdavidsaver]

Work in progress porting from the pvAccessCPP library to the new PVXS. This should be a functionally transparent change with the exception of p4p.*.raw.

Present state:

All features ported, and unit tests passing.

• [X] p4p Value
• [X] p4p.client.raw
  • Change to monitor callback. Disconnected and RemoteError now delivered through Subscription.pop().
• [X] p4p.client.thread
• [X] p4p.client.cothread
• [X] p4p.client.asyncio
• [X] p4p.server
• [X] p4p.server.asyncio
• [X] p4p.server.thread
• [X] p4p.server.cothread
• [X] p4p.server.Qt
• [X] p4p.gw

[mdavidsaver]

Depends on 'master' branch from https://github.com/mdavidsaver/pvxs

[mdavidsaver]

At long last, all features have been ported (I think), and the unit tests are passing.