Adding test of EVCS transaction handling

EVCS = ExpectedValueCheckingStore and associated manager and
transaction classes.  These classes hold a transaction which is
supposed to provide global strong consistency and another transaction
which does not guarantee any particular level of consistency.  This
new test checks that the strong consistency tx is used when reading
data to check expectations and when writing mutations for a
transaction which has taken at least one lock.
Make txs w/ locks write at QUORUM or higher

Along with cb8660102019c44288ee5556a8851b4307a211da and the tests
added by 7b159ef16be02f018053f6d0e0efa32809cdd8a9, this commit fixes
strong-consistency transaction both to read actual values associated
with expectations out of the backing store and to mutate data on the
backing store when at least one lock is held.

This commit also removes the warning added in
cb8660102019c44288ee5556a8851b4307a211da since it's now obsolete.