MEDIUM: stick-table: touch updates under an upgradable read lock

MEDIUM: stick-table: touch updates under an upgradable read lock

Instead of taking the update's write lock in stktable_touch_with_exp(),
while most of the time under high load there is nothing to update because
the entry is touched before having been synchronized present, let's do
the check under a read lock and upgrade it to perform the update if
needed. These updates are rare and the contention is not expected to be
very high, so at the first failure to upgrade we retry directly with a
write lock.

By doing so the performance has almost doubled again, from 1140 to 2050k
with a peers section enabled. The contention is now on taking the read
lock itself, so there's little to be gained beyond this in this function.