Poller coalesces multiple subscriptions to the same cell.
Methods to get the size of the poller table.

This should eventually become a reported measurement to check for leaks.
Define 'loose' cells which have reliable subscription without polling.
Test poller unsubscription.
Use LooseCell subscriptions to eliminate tune_hook.
Allow configuration file to do asynchronous things.
Use LooseCell in HamlibRig.
AudioSource can be configured to take its center frequency from a supplied cell.

This allows the frequency to be correctly reported and edited when the
audio is from an external SSB or I/Q receiver controlled e.g. through
hamlib.
Remove audio source rebuilding since the bug is fixed.

Bug fixed in gnuradio 3.7.3: <http://gnuradio.org/redmine/issues/623>