It turns out we were only notifying plugins of freezer commits and
cleaning up our record when the block being processed was greater
than MAX_UINT64.
So, uh, never.
We had been assuming that the `item` returned from batch.commit()
was the item committed, but it's actually the next item to be added
to the freezer, and multiple items can be committed in a single batch.
This commit finds the smallest item in the freezer and iterates from
that to the number returned by commit(), passing any tracked blocks
in that range to plugins.
This commit adds a ModifyAncients hook that plugins can implement
to more accurately track what Geth is doing under the hood. We
still support the old AppendAncients interface as best we can,
though internal changes may make it so that it does not behave
as it once did.
