Centralize shared executors

patch by Sam Tunnicliffe; reviewed by Aleksey Yeschenko for
CASSANDRA-8055
Merge branch 'cassandra-2.1' into trunk

Conflicts:
	src/java/org/apache/cassandra/cql3/QueryProcessor.java
	src/java/org/apache/cassandra/db/ColumnFamilyStore.java
	src/java/org/apache/cassandra/db/HintedHandOffManager.java
	src/java/org/apache/cassandra/io/sstable/format/SSTableReader.java
	src/java/org/apache/cassandra/service/StorageService.java
	test/unit/org/apache/cassandra/db/KeyCacheTest.java
improve JBOD disk utilization
Merge branch 'cassandra-2.1' into trunk

Conflicts:
	src/java/org/apache/cassandra/db/compaction/CompactionManager.java
	src/java/org/apache/cassandra/db/compaction/Scrubber.java
	src/java/org/apache/cassandra/service/StorageService.java
	src/java/org/apache/cassandra/streaming/StreamReader.java