Validate that collection values are < 64K

patch by slebresne; reviewed by jbellis for CASSANDRA-5355
Merge branch 'cassandra-1.2' into trunk
Replace compaction lock with runWithCompactionsDisabled
patch by jbellis; reviewed by slebresne for CASSANDRA-3430
Improve LeveledScanner work estimation
patch by Marcus Eriksson; reviewed by jbellis for CASSANDRA-5250
Add UseTLAB JVM Opt

Patch by tjake; reviewed by Ryan McGuire for CASSANDRA-5361
merge from 1.1
cqlsh: Remove ASSUME command

patch by Aleksey Yeschenko; reviewed by Brandon Williams for
CASSANDRA-5331
Change Kernel Page Cache skipping into row preheating (disabled by default)
patch by Pavel Yaskevich; reviewed by Jonathan Ellis and Yuki Morishita for CASSANDRA-4937
merge with 1.2