Bluetooth: Mesh: Fix Friend messaging state tracking

There were several issues with the code:

 - queue_size wasn't properly kept up to date, leading to erroneous
   buffer discarding logic.

 - Poll timeout when there were buffers in the Friend Queue didn't
   work because we didn't track if there'd been a preceding request
   for messages or not (hence the added pending_req variable).

 - We would overwrite the recv_delay timer if there was another
   request while the previous one was still sending (a likely scenario
   if we send out multiple advertising events per packet).

 - We weren't canceling the sending of a buffer if the Friendship was
   suddenly cleared.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>