Prevent NPE on FD conviction race

Patch by brandonwilliams, reviewed by thobbs for CASSANDRA-7940
Fix RowIndexEntry to report correct serializedSize
patch by Pavel Yaskevich; reviewed by Aleksey Yeschenko for CASSANDRA-7948
Include schema_triggers CF in readable system resources

patch by Adam Holmberg; reviewed by Aleksey Yeschenko for CASSANDRA-7967
Allow permissions cache to be set via JMX

Patch by brandonwilliams, reviewed by aleksey for CASSANDRA-7968
Log unclean client disconnect excs at DEBUG

Patch by Graham Sanderson; reviewed by Tyler Hobbs for CASSANDRA-7849