Reduce the amount of database access for HAVE_PRI_SERVICE_MESSAGES.

Rework HAVE_PRI_SERVICE_MESSAGES to not use the active values directly
from the database.  Database access is likely expensive.  Database access
now only happens on initialization, destruction, and when the B channel is
taken in or out of service.

This change is not related to call waiting but it would cause the search
for a call waiting interface to be very expensive and slow down D channel
message servicing.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@251538 65c4cc65-6c06-0410-ace0-fbb531ad65f3