Don't send all debug messages to Telepathy

The log handler can be invoked at bad times, and in particular
it can be invoked from gsignal with the signal lock taken.
At that time, calling into arbitrary high-level APIs can
cause a dead-lock.
Instead, only send to telepathy the tp-glib debug messages.
Everything else is in the journal anyway.

https://bugzilla.gnome.org/show_bug.cgi?id=724256