agent: fix tcp connection to turn servers

agent: fix tcp connection to turn servers

A previous commit broke the logic used to start a discovery request for
tcp turn servers. The ambiguity came from the distinction between the
type of the turn server (turn->type), the compatibility of the
transport of the local base candidate (turn_tcp), and the reliability
of the underlying tcp socket (reliable_tcp).

reliable_tcp indicates whether the turn allocate request should be
"framed" in a tcp packet, according to RFC 4571. This is required in
OC2007R2 only.

This commit also puts the setup of the tcp turn socket in a separate
function, because such setup is also required when handling
try-alternate (code 300) stun errors on these tcp sockets, where we have
to setup a new connection to another tcp turn server.