Catch memtable flush exceptions during shutdown
patch by Sergio Bossa; reviewed by jbellis for CASSANDRA-6735
Completer shall always return a collection
Document that TTL <= 0 means no TTL
update changes
r/m populate_io_cache_on_flush from yaml (per-table since #4694)
Add CMSClassUnloadingEnabled JVM option
Patch by Jonathan Lacefield, reviewed by brandonwilliams for
CASSANDRA-6541
Fix resetAndTruncate:ing CompressionMetadata

Patch by kvaster, reviewed by marcuse for CASSANDRA-6791