[spam][crazy][ot] log: #3 write-anywhere to an append-only-log

[Undiscussed Horrific Abuse, One Victim of Many]

Flush:
  - an updatable tree containing writes and indexes, designed to be
useful for syncing with a storage medium. contains a list of
subindexes
  SubIndex
    - a list of bounded regions (flush entries) of previous flushes,
each held as a useful tree. contains links to its left and write to
new writes contained in the flush.