Use a socket for communication between agent and bagpipe-bgp

[Thomas Morin]

Currently, local TCP on 127.0.0.1 is used to let the agent extension connect to bagpipe-bgp API.

We want to better restrict the access to this API.

The proposal is to:
a) allow bagpipe-bgp to listen to a unix socket (with a configuration so that nothing, or perhaps only the looking-glass is exposed on TCP)
and b) have the agent extension be configurable to use this unix socket instead of TCP

(b) seems to imply using another module than "httplib2", since httplib2 does not seem to support Unix sockets