Move out accept() code into its own thread. This removes annoying
ability for one client that connects to the control socket and just
hangs onto that connection without sending any data to block other
clients trying to talk to the same socket. In addition, it adds
possibiluty for the local socket client to reduce overhead by keeping
the connection open instead of going through connect(), write(),
close() cycle on every command, which might be imporant to handle
high-CPS traffic loads. This works fine with our python client code,
but needs more testing to ensure this won't screw up existing clients,
i.e. opensips, kamalio and such, in which case we would make it opt-in
feature, to be made default in 3.0 or so.

This also makes it relatively easy to add support for TCP control
channel as well as for multiple stream-based control sockets (i.e.
tcp+unix domain and or several tcp/several unix domain at the same
time).
ensure this won't screw up existing clients