extension_state: Add new extension state API.

extension_state: Add new extension state API.

Extension state to this point has been an API implemented
inside the PBX core resulting in its state being intermingled
with that of the dialplan. This increased the complexity of
the PBX core and made it difficult to enact improvements.

This change adds a new separate extension state API
which receives updates from the PBX core as configuration
changes but maintains its own separate state. The API is
also written to fully take advantage of modern APIs in a
more selective manner by subscribing each extension state to
only the devices it is interested in, ultimately reducing
resource consumption during updates. Presence state updates
being infrequently done use a single shared subscription that
goes through the extension states to find and update ones
that the update is applicable to.

Legacy API support is provided by reimplementing the API
as wrappers over the new extension state API. This improves
the legacy API by making it multithreaded, with each callback
being individually subscribed.

Autohints support is maintained but has been separated out
into a self contained new implementation.

Synchronous subscription support has also been added to
Stasis to remove the overhead of asynchronous publishing when
the handling of published messages is small and fast.