Segregate watchlist and recentshangeslinked queries on all shards.

As happened with s1 enwiki long ago, these query types are worth
separating from general traffic on the large wikis to reduce the
effects of hitting cold data. Not really necessary on s3 but nice
to be consistent.

Also the logpager slaves now have partitioned logging tables on
a hash of log_user. All these queries that filter by user id can
take advantage of partition pruning there.

Change-Id: I5f2816468695e5ff7cd61886482f8d4a214fd670