Use atomic functions to manipulate `rev` of `fakeRevGetter`
so that the tester goroutine can update the
value without race with the compactor's goroutine.
Perviously, we advance checkCompactionInterval more than we should.
The compaction might happen nondeterministically since there is no
synchronization before we call clock.Advance().
The number of rg.Wait() should be equal to the number of Advance() if
compactor routine and test routine run at the same pace. However, in our current
test, we call Advance() more than rg.Wait().
It works OK when the compactor routine runs "slower" than the test routine, which
is the common case. However, when the speed changes, the compactor routine might
block rg.Rev() since there is not enough calls of rg.Wait().
This commit forces the compactor and test routine to run at the same pace. And we supply
the exact number of Advance() and wg.Wait() that compactor needs.
If the test calls clock.Advance() after the compactor checks clock.Now()
but before the compactor calls clock.After(), the compactor will wait
forever on clock.After() expecting the lost clock.Advance().
Reproduced failure by putting a Sleep() in the clock.Now() continue path.
Fixes#6060 (again)
