serializer: move/add asterisk serializer pool functionality

Serializer pools have previously existed in Asterisk. However, for the most
part the code has been duplicated across modules. This patch abstracts the
code into an 'ast_serializer_pool' object. As well the code is now centralized
in serializer.c/h.

In addition serializer pools can now optionally be monitored by a shutdown
group. This will prevent the pool from being destroyed until all serializers
have completed.

Change-Id: Ib1e906144b90ffd4d5ed9826f0b719ca9c6d2971