stasis: Fix crash at shutdown.

When compiling in dev mode, stasis statistics are enabled and can cause
a crash at shutdown due to the following:
- Containers are freed
- Topics and subscriptions remain
- When those topics and subscriptions are deallocated, they go to do
  things with the container

This changes the containers to global ao2 objects, and whenever needed
in the code, a reference must be obtained and checked before any
operations can be done.

ASTERISK-28353 #close

Change-Id: Ie7d5e907fcfcb4d65bd36d5e4eb923126fde8d33