[ot][spam][crazy] crazylog: append-only random-access data

[Undiscussed Horrific Abuse, One Victim of Many]

each flush region has a left write and a right write. these are passed
to the constructor.

a flush can happen two ways:
1 - new flush. starts with nothing in it. adds only writes.
2 - old flush. starts with a previous flush and no writes. new writes
can break up the reference to the previous flush.