Preemptive open of compaction results

Patch by benedict; reviewed by marcuse for CASSANDRA-6916
Merge branch 'cassandra-2.1' into trunk

Conflicts:
	src/java/org/apache/cassandra/cql/AlterTableStatement.java
	src/java/org/apache/cassandra/cql/CreateColumnFamilyStatement.java
	test/unit/org/apache/cassandra/db/ColumnFamilyStoreTest.java