Correctly validate sparse composite cells
patch by Tyler Hobbs; reviewed by jbellis for CASSANDRA-5855
merge from 1.1
merge from 1.2
Merge branch 'cassandra-2.0.0' into cassandra-2.0
Merge remote-tracking branch 'origin/cassandra-2.0' into cassandra-2.0
Switch from crc32 to adler32 for compressed sstable checksumns
and change to checksum the post-compressed data

Patch by tjake; reviewed by jbellis for (CASSANDRA-5862)