Add checksum to compressed index.

Summary:
 - Add CRC32 checksum for memsector data.

closes #29.

Test Plan:
mws-index -I data/zbl -o data/zbl-compressed
mwsd-load -I data/zbl-compressed

Reviewers: radu.hambasan

Reviewed By: radu.hambasan

Differential Revision: http://phab.code4fun.de/D62