mostly because they rely on closing and re-opening the DB

generated by unit tests

comment even says so

if something goes wrong with this test

need to call lockMeta before we synchronize on the Database object or
we get an ABBA deadlock

the problem was that the first database connection closing would
sometimes lead to the database being deleted before the other
connections got going

Fix TestClearReferences

TestClearReferences currently fails because the fields in
LocalDateTimeUtils are not final.

mostly because various tests fail in (mvStore && !mvcc) mode, was
previously hidden by the fact that we did not test that combination,
despite it being our default mode

so we don't use up too much in the way of Travis/CI resources

Also don't bother testing MVCC mode, since that's going to go away at
some point, and it's certainly not a default setting for us