Initial implementation of metadata log sharing.

Initial implementation of metadata log sharing.

Allow metadata written to segments to be reused between snapshots.  Keep
track of what metadata was written out on the client, and when identicial
metadata would be written on a subsequent backup, instead emit a reference
to the old metadata.

This needs more testing and verification.  There also needs to be a
mechanism for performing the equivalent of segment cleaning for metadata,
so that the metadata log does not become excessively fragmented over time.