Read message id as string from older nodes.

Patch by marcuse, reviewed by jbellis for CASSANDRA-6840
Merge branch 'cassandra-2.0' into cassandra-2.1
Fix schema concurrency exceptions
patch by Benedict Elliott Smith; reviewed by Carl Yeksigian for CASSANDRA-6841
Race condition in Batch CLE

patch by belliottsmith reviewed by dbrosius for cassandra-6860
merge from 2.0
Merge remote-tracking branch 'origin/cassandra-2.1' into cassandra-2.1