tabletserver: execution plan change

It appears that the MariaDB optimizer is not good at handling
tuple equalities for update statements. This cl changes the exection
plan of all tuple equalities from:
where (pk1, pk2) = (:v1, :v2) or (pk1, pk2) = (:v3, :v4) to
where (pk1 = :v1 and pk2 = :v2) or (pk1 = :v3 and pk2 = :v4), etc.
revert accidental Makefile change
v1 of writeMastersOnly strategy for split_clone workers
Passing tests for v1 of writeMastersOnly strategy
Break out TestSplitClone into multiple tests
Modify sharding integration tests for vtworker to use writeMastersOnly strategy
Make vtworker find master tablets more efficiently when using writeMastersOnly strategy
Make writeMastersOnly into a proper parsed strategy
Merge pull request #153 from youtube/aaijazi_write_masters_only_strategy

writeMastersOnly strategy for split_clone workers
Adding unit test for vtctl client-server protocols. Using it for gorpc.
Making the test protocols easier to select with a command line parameter.

(based on mysql_flavor stuff).
Merge pull request #157 from youtube/suguwork

Suguwork
Add ExternalID parameter to TabletExternallyReparented.

This ID can optionally be provided by the external tool doing the
reparnt. The ID gets printed in logs to allow cross-referencing with
logs from the external tool.
Fixing missed inheritence.
Merge pull request #160 from youtube/resharding

Making the test protocols easier to select with a command line parameter...
Merge pull request #159 from youtube/replication

Adding unit test for vtctl client-server protocols. Using it for gorpc.
Merge pull request #161 from youtube/tablet-externally-reparented

Add ExternalID parameter to TabletExternallyReparented.
Make topo server flavor pluggable in integration tests.
Remove extra copy of zk_setup().
Merge pull request #162 from youtube/topo-server-flavor

Make topo server flavor pluggable in integration tests.