bluetooth: controller: Split events into normal and priority

For the host to be able to block on the recv_thread while waiting for a
number of completed packets HCI event, those are pulled from the radio
in a new, higher priority thread that also schedules the execution of
the lower priority event/ACL data recv_thread.

Change-id: I9d356bd297d0504cb16a032fb5fe5530693546e2
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>
Signed-off-by: Vinayak Chettimada <vinayak.kariappa.chettimada@nordicsemi.no>