set hosts_max_try_hardlimit to avoid excessive retries with gmail

set hosts_max_try_hardlimit to avoid excessive retries with gmail

Per the exim manual:

  "...when the hosts_max_try limit is reached, Exim looks down the
  host list to see if there is a subsequent host with a
  different (higher valued) MX. If there is, that host is considered
  next..."

Gmail seems to return sets of records with different priorities
now (or maybe always did), and exim is now checking all of them
instead of bailing out early.

Set the hardlimit to 3; this lets us make one attempt at delivering to
a lower priority mx without hammering gmail excessively.