Initial support for efficient sub-file incrementals.

This is a cleaned-up version of the code written for the OSDI'08
submission to implement sub-file incremental deltas.  Large files are
broken into small chunks (in a content-sensitive manner with Rabin
fingerprints, as in LBFS).  Hash values are computed for the chunks and
stored in a new database table in the local database.  On subsequent
backups, when a file has changed, search for chunks that are identical,
so that portions of old blocks may be re-used even if the entire blocks
cannot be.