simplify queue's basic_consume handler
backported from bug23749 branch
- the call to update_ch_record in the is_ch_blocked(C1) == false
  branch was superfluos since the preceding update_consumer_count
  calls update_ch_record
- all the checking whether the channel is blocked, and associated
  branching was just an optimisation. And not a particularly important
  one, since a) the "a new consumer comes along while its
  channel is blocked" case is hardly on the critical path, and b)
  exactly the same check is performed as part of run_message_queue (in
  deliver_msg_to_consumer/3). So get rid of it.