Shutdown batchlog executor in SS#drain()

patch by Sergio Bossa; reviewed by Aleksey Yeschenko for CASSANDRA-7025
Merge branch 'cassandra-1.2' into cassandra-2.0
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	src/java/org/apache/cassandra/service/StorageService.java