client: Hold presses in queues, one queue per send_count

This lets us efficiently hold many more presses with pending resends
and eliminates the failure mode in which young presses could be dropped
because the sleeps_heap was full of old presses.

Instead of storing each press in the sleeps_heap, we now only store up
to config_resend_count entries in the sleeps_heap.

Old presses now only compete with similarly-old presses for retention
space.