[libnice] keepalive connchecks for TCP candidates

[Harald Glanzer]

hi,

for my appliation (farstream + libnice) i need some way to detect network failures, i.e. i need
a way to get signalled whenever the _remote_ side disappears.

it turned out that using binding requests as keepalive mechnism works fine for UDP candidates. whenever the remote
side has network issues and disappears, the local side does some retries and sends a signal which i am using to close the
session from the application side.

nevertheless, if TURN with TCP candidates is utilized, no binding requests are created (agent/conncheck.c, line ~850),
and therefore, i do not get notified that the transport channel is not active any more. i guess this happens because the local connection to 
the TURN server itself is still active, but the _remote_ endpoint has gone away (which, it seems, is not recognized by the TURN 
server, who does not close the connection to the local TURN client).

i am now struggling with the question if it makes sense to use keepalive binding requests as connection-checks for TCP too?
i wonder if the TURN LIFETIME attribute ( https://tools.ietf.org/html/rfc5766#section-14.2 ) would be suitable, and if libnice is able
to use this feature (some basic tests regarding LIFETIME were not sucessfull..)

thx in advance
hari