Eliminate "seen" Balking

Replaces the current "seen" balking with the original "threads"
implementation. The former is too overzealous since it means that if the
network connection goes down, whatever TTS tag that was being attempted
will never be attempted again until the session is restarted.
Merge bug/overzealous-balking

This loosens the requirement that a particular TTS tag will only be
attempted once and makes it so only ONE THREAD will ever be running for
a given TTS tag at any given time, which allows for network outages.