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>