Support persistent socket connection to core-agent

[asgrim]

At the moment, the client library handles connecting to the core agent socket server. It would be quite handy if we could move this into the extension and open a persistent socket to do this instead.

(note: what interactions in NTS/ZTS modes do we need to consider?)
(note 2: if two apps are using the same running PHP server, do they share the same socket?)

Potential other uses/optimisations also become available, such as sending app metadata once instead of every request.