MEDIUM: stick-table: switch to rdlock in stktable_lookup() and lookup_key()

MEDIUM: stick-table: switch to rdlock in stktable_lookup() and lookup_key()

These functions do not modify anything in the the table except the refcount
on success. Let's just lock the table for shared accesses and make use of
atomic ops to update the refcount. This brings a nice gain from 425k to
455k under 48 threads (7%), but some contention remains on the exclusive
locks in other parts.

Note that the refcount continues to be updated under the lock because it's
not yet certain whether there are races between it and some of the exclusive
lock on the table. The difference is marginal and we prefer to stay on the
safe side for now.