dm vdo: implement the chapter volume store

The volume store structures manage the reading and writing of chapter
pages. When a chapter is closed, it is packed into a read-only structure,
split across several pages, and written to storage.

The volume store also contains a cache and specialized queues that sort and
batch requests by the page they need, in order to minimize latency and I/O
requests when records have to be read from storage. The cache and queues
also coordinate with the volume index to ensure that the volume does not
waste resources reading pages that are no longer valid.

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>
Co-developed-by: John Wiele <jwiele@redhat.com>
Signed-off-by: John Wiele <jwiele@redhat.com>
Signed-off-by: Matthew Sakai <msakai@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>