When a connection termiates abruptly while is it the principal_conn, make sure it...

When a connection termiates abruptly while is it the principal_conn, make sure it sets the principal_conn
to NULL and cleans up the bonjour flags, if appropriate.

Simplify the TEARDOWN handlers and the thress teardown functions by incporporating
the above code in the teardown_phase_two (for AP2) and teardown (fpr AP1) functions.

It means that closing a connection will block on the principal_conn_lock, so if
you have the principal_conn_lock, closing will not complete until you release it.

Maybe we need a principal_conn_acquisition_lock for that...