Flush the batch when exceeding a certain size, instead of when reaching a number
of batched operations.
Move batch to lowlevel to be able to use it in NamespacedKV.
Increase the leveldb memory buffer from 4 to 16 MiB.
There was a problem in iterating the sequence index that could result
in missing updates. The issue is that while the index was (correctly)
iterated in a snapshot, the actual file infos were read dirty outside of
the snapshot. This fixes this by doing the reads inside the snapshot,
and also updates a couple of other places that did the same thing more
or less harmfully (I didn't investigate).
To avoid similar issues in the future I did some renaming of the
getFile* methods - the ones in a transaction are just getFile, while the
ones directly on the database are variants of getFileDirty to highlight
what's going on.
