blob: fix multistage write for immutable blobs

Now we can't simply overwrite data in-place because there is no binlog
for data modifications. On each overwrite in `plain_write` and
`write_commit` (if it's not in the last base and thus is subject to
data-sort) we must preform copy of previous record.
blob: improved logging in multi-stage write
blob: fix copying of old record on prepare