common/SloppyCRCMap: add type to sloppily track crcs

Signed-off-by: Sage Weil <sage@inktank.com>

sloppy
os/FileStore: add sloppy crc tracking

Opportunistically track CRCs for data we write and verify it for data
we read.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #660 from ceph/wip-fs-crc

sloppy / opportunistic CRC tracking in the filestore

Reviewed-by: Samuel Just <sam.just@inktank.com>