stasis: Add setting subscription congestion levels.

Stasis subscriptions and message routers create taskprocessors to process
the event messages.  API calls are needed to be able to set the congestion
levels of these taskprocessors for selected subscriptions and message
routers.

* Updated CDR, CEL, and manager's stasis subscription congestion levels
based upon stress testing.  Increased the congestion levels to reduce the
potential for bursty call setup/teardown activity from triggering the
taskprocessor overload alert.  CDRs in particular need an extra high
congestion level because they can take awhile to process the stasis
messages.

ASTERISK-26088
Reported by:  Richard Mudgett

Change-Id: Id0a716394b4eee746dd158acc63d703902450244