CLEANUP: stream: lookup server ID using standard functions

CLEANUP: stream: lookup server ID using standard functions

The server lookup in sticking_rule_find_target() uses an open-coded tree
search while we have a function for this server_find_by_id(). In addition,
due to the way it's coded, the stick-table lock also covers the server
lookup by accident instead of being released earlier. This is not a real
problem though since such feature is rarely used nowadays.

Let's clean all this stuff by first retrieving the ID under the lock and
then looking up the corresponding server.