Make HOLD notification blocking; only do multi-reads when closing connection.

. Make the HOLD notifications blocking, to close a potential window
  where a HOLD message has been queued to the listener, but not yet
  received because the listener is busy reading from one or more
  very active connections. This ensures that a received response will
  ALWAYS have its handling info available, unless the respones has
  timed out.

. Fix yesterday's change for receiving full messages on hangup (which
  are likely to be UNSOLICITEDSTATUS messages with error info). Always
  reading connections until exhausted could lead to the listener
  spending excessively large amounts of time reading between checking
  its command queue, leading to responses arriving with
  not-yet-processed HOLD messages in-queue and getting flagged as
  unexpected.

. Also, add some additional constraints to ensure the listener's
  msg and rx_info structs are managed properly.