Replace TestTxApplicationSychronization.

TestTxApplicationSynchronization no longer works and has resisted our attempts
to debug it. This extremely complex and arcane test was originally introduced to
reproduce a race condition; the race condition was fixed by ensuring that commands
are applied in the same order whether or not we are recovering. We have therefore
replaced the broken test with a simpler test that specifically enforces that
ordering (WriteTransactionCommandOrderingTest).