astobj2-ify the SLA code.

The SLA code within app_meetme was written before asotbj2 had been
merged into Asterisk.  Worse, support for reloads did not exist at first
and was added later as a bolt-on feature.  I knew at the time that
reloading was not safe at all while SLA was in use, so the reload would
be queued up to execute when the system was idle.  Unfortunately, this
approach was still prone to errors beyond the fact that this was the
only place in Asterisk where configuration was not reloaded
instantly when requested.

This patch converts the base objects, sla_trunk and sla_station, to be
reference counted objects using astobj2.  This allows reloads to be
processed while the system is in use.  The code ensures that the objects
will not disappear while one of the other threads is using them.
However, they will be immediately removed from the global trunk and
station containers so no new calls will use them if removed from
configuration.