tabletserver: DML workaround for MariaDB

It turns out that MariaDB's optimizer triggers
a full table scan if the number of rows affected by a DML
exceeds a rather low amount. So, we need to break up our
updates into multiple chunks if we need to modify too many
rows. Default is set at 500.
Merge pull request #171 from youtube/suguwork

tabletserver: DML workaround for MariaDB
tabletserver: rewrite max dml rows test

The previous test was slapped together to make sure the
feature worked. This is a more robust version of the same
test.
Merge pull request #172 from youtube/suguwork

tabletserver: rewrite max dml rows test
tabletserver: better error messages.
Merge pull request #173 from youtube/suguwork

tabletserver: better error messages.
Merge pull request #170 from youtube/vtclient

Pruned the query out of the integrity error.
Grammar/typo correction
Merge pull request #177 from flimzy/doc

Grammar/typo correction
Explicit conversion of exception args to tuple.