Separate responsibilities of Adhearsion.active_calls into a passive collection and an active supervisor

This allows for greater stability (the actor portion is a simple supervisor which is crash-resistant) and greater performance (call lookups are now synchronised by a simple Monitor, rather than an actor, which is much faster).
Merge branch 'refs/heads/develop' into feature/passive_calls_collection
[BUGFIX] Specs expect active_calls itself to be an actor
Merge branch 'refs/heads/develop' into feature/passive_calls_collection
Merge branch 'feature/passive_calls_collection' into develop