MINOR: stick-table: do not take an exclusive lock when downing ref_cnt

MINOR: stick-table: do not take an exclusive lock when downing ref_cnt

At plenty of places we decrement ts->ref_cnt under the write lock
because it's held. We don't technically need it to be done that way
if there's contention and an atomic could suffice. However until all
places are turned to atomic, we at least need to do that under a
read lock for now, so that we don't mix atomic and non-atomic uses.
Regardless it already brings ~1.5% req rate improvement with 3 trackers
on the same table under 48 threads at 184k->187k rps.