Compute checksums of segments and store them in the local database.

Compute checksums of segments and store them in the local database.

When a segment is fully written out, compute a checksum of the file
actually written (post-filtering).  Store this in the local database so
that it will be possible to write out, at the end of a backup, a file
containing the checksums of all segments used for the snapshot (including
old ones not written out in this execution).