Clean up IndexInfo on keyspace/table drops

patch by Sam Tunnicliffe; reviewed by Aleksey Yeschenko for
CASSANDRA-6924
Merge branch 'cassandra-2.1' into trunk
Snapshot only related SSTables when sequential repair

patch by yukim; reviewed by jmckenzie for CASSANDRA-7024
Merge branch 'cassandra-2.1' into trunk
Allow cassandra to compile under java 8

patch by dbrosius reviewed by jmckenzie for cassandra-7028