BUG/MEDIUM: stick-tables: don't loop on non-expirable entries

BUG/MEDIUM: stick-tables: don't loop on non-expirable entries

The stick-table expiration of ref-counted entries was insufficiently
addresse by commit 324f0a60ab ("BUG/MINOR: stick-tables: never leave
used entries without expiration"), because now entries are just requeued
where they were, so they're visited over and over for long sessions,
causing process_table_expire() to loop, eating CPU and causing lock
contention.

Here we take care of refreshing their timeer when they are met, so
that we don't meet them more than once per stick-table lifetime. It
should address at least a part of the recent degradation that Felipe
noticed in GH #3084.

Since the fix above was marked for backporting to 3.2, this one should
be backported there as well.