MEDIUM: stick-tables: give up on lock contention in process_table_expire()

MEDIUM: stick-tables: give up on lock contention in process_table_expire()

process_table_expire() can take quite a lot of time running over all
shards. During this time it will hinder track-sc rules and peers, which
will experience an increased latency to do their work, especially peers
where each message will cause a lock, whose cumulated time can exceed
the watchdog's patience.

Here, we proceed just like in stktable_trash_oldest(), which is that
we're using a trylock to detect contention. The first time it happens,
if we hadn't purged anything, we switch to a regular lock to perform
the operation, and next time it happens we abort. This guarantees that
some entries will be expired and that contention will be reduced with
when detected.

With this change, various tests didn't manage to produce any warning,
including at the end of the load generation session.

This should be backported to 3.2 after a bit more testing.