Having one boolean per reviewdb table being replaced is overkill; in
practice, even during the migration, even on googlesource.com, we are
never going to turn on reading from some Change sub-tables but not
others. However, we will very likely turn on writes for changes before
writes for, say, accounts, so we do need some granularity.
Explain the supported level of granularity in the NotesMigration
javadoc.
Change-Id: I98261fa888b3f6b71ed896e6c791aeee8324e898