Categorize upserts as PASS_DML

Upserts are not safe for statement based replication:
http://bugs.mysql.com/bug.php?id=58637
So, vttablet now categorizes it as pass-though DML,
which will eventually get rejected because of
update stream & rowcache.
In the future, we can allow a 'dumb mode' where
PASS_DML plans are allowed if people don't care
about rowcache or update stream.
Add comment about why upserts are unsafe
Merge branch 'monitoring'
Merge branch 'replication'