Now, instead of reverting only the first block on powerloss, _all_
blocks since the last sync are reverted (except the in-flight block, if
you reverted that it would be the same as noop powerloss).
It was a bit frustrating trying to reproduce known holes in our sync
logic before this, but reverting all blocks really is the worst case,
so we should have quite a bit more confidence going forward.
This was a bit tricky to implement without memory leaks everywhere,
since we need to be able to resume for exhaustive powerloss testing. But
emubd's copy-on-write block emulation really shines here.
Christopher Haster
780527d00a