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.
This adds a thin type that holds the state associated with the
leveldb.DB, leaving the huge Instance type more or less stateless. Also
moves some keying stuff into the DB package so that other packages need
not know the keying specifics.
(This does not, yet, fix the cmd/stindex program, in order to keep the
diff size down. Hence the keying constants are still exported.)
