dm vdo: implement the open chapter and chapter indexes

Deduplication records are stored in groups called chapters. New records are
collected in a structure called the open chapter, which is optimized for
adding, removing, and sorting records.

When a chapter fills, it is packed into a read-only structure called a
closed chapter, which is optimized for searching and reading. The closed
chapter includes a delta index, called the chapter index, which maps each
record name to the record page containing the record and allows the index
to read at most one record page when looking up a record.

Co-developed-by: J. corwin Coburn <corwin@hurlbutnet.net>
Signed-off-by: J. corwin Coburn <corwin@hurlbutnet.net>
Co-developed-by: Michael Sclafani <dm-devel@lists.linux.dev>
Signed-off-by: Michael Sclafani <dm-devel@lists.linux.dev>
Co-developed-by: Thomas Jaskiewicz <tom@jaskiewicz.us>
Signed-off-by: Thomas Jaskiewicz <tom@jaskiewicz.us>
Signed-off-by: Matthew Sakai <msakai@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>