res_stasis: signal when new command is queued

res_stasis: signal when new command is queued

res_statsis's app loop sleeps for up to .2s waiting on input
to a channel before re-checking the command queue. This can
cause delays between channel setup and bridge.

This change is to send a SIGURG on the sleeping thread when
a new command is enqueued. This exits the sleeping thread out
of the ast_waitfor() call triggering the new command being
processed on the channel immediately.

Resolves: #362

UserNote: Call setup times should be significantly improved
when using ARI.