Refactor DNS script's state management to improve performance.

The amount of timers involved in DNS::PendingMessage tables'
expiration attributes have a significant performance hit.  Instead the
script now relies solely on maximum thresholds for pending message
quantities to limit amount of accumulated state.  There's a new option,
"DNS::max_pending_query_ids", to limit the number outstanding messages
across all DNS query IDs ("DNS::max_pending_msgs" still limits number
of outstanding messages for a *given* query ID).
Merge remote-tracking branch 'origin/topic/jsiwek/dns-perf'

* origin/topic/jsiwek/dns-perf:
  Refactor DNS script's state management to improve performance.

BIT-1133