Make read "defrag" async to reclaim memtables

patch by benedict; reviewed by jbellis CASSANDRA-8459
Merge branch 'cassandra-2.1' into trunk
Ensure memtable flush cannot expire commit log entries from its future

patch by benedict; reviewed by aweisburg for CASSANDRA-8383
Merge branch 'cassandra-2.1' into trunk

Conflicts:
	src/java/org/apache/cassandra/db/DataTracker.java
 Reduce cassandra-stress sampling memory requirements

patch by benedict; reviewed by branimir for CASSANDRA-7926
Merge branch 'cassandra-2.1' into trunk
cassandra-stress simultaneous inserts over same seed

patch by benedict; reviewed by rstupp CASSANDRA-7964
Merge branch 'cassandra-2.1' into trunk
Fix NPE after dropping a keyspace

Patch by tjake; reviewed by Chris Lohfink for CASSANDRA-8332
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	CHANGES.txt
Merge branch 'cassandra-2.1' into trunk
Scale memtable slab allocation logarithmically

patch by benedict; reviewed by jay patel for CASSANDRA-7882
Merge branch 'cassandra-2.1' into trunk

Conflicts:
	src/java/org/apache/cassandra/utils/memory/NativeAllocator.java
cassandra-stress simultaneous inserts over same seed
(take two)

patch by benedict; reviewed by rstupp CASSANDRA-7964
Merge branch 'cassandra-2.1' into trunk
ninja fixup
Fix NPE when writetime() or ttl() are nested inside a fn call

Patch by Tyler Hobbs; reviewed by Benjamin Lerer for CASSANDRA-8451
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	CHANGES.txt
	src/java/org/apache/cassandra/cql3/statements/Selection.java
Merge branch 'cassandra-2.1' into trunk

Conflicts:
	src/java/org/apache/cassandra/cql3/statements/Selection.java
Avoid stack overflow on large clustering IN values

Patch by Tyler Hobbs; reviewed by Benjamin Lerer for CASSANDRA-8410