MEDIUM: stick-table: make stktable_get_entry() look up under a read lock

MEDIUM: stick-table: make stktable_get_entry() look up under a read lock

On a 24-core machine doing lots of track-sc, it was found that the lock
in stktable_get_entry() was responsible for 25% of the CPU alone. It's
sad because most of its job is to protect the table during the lookup.

Here we're taking a slightly different approach: the lock is first taken
for reads during the lookup, and only in case of failure we switch it for
a write lock. We don't even perform an upgrade here since an allocation
is needed between the two, it would be wasted to do it under the lock,
and is generally not a good idea, so better release the read lock and
try again.

Here the performance under 48 threads with 3 trackers on the same table
jumped from 455k to 2.07M, or 4.55x! Note that the same approach should
be possible for stktable_set_entry().