Optimize auth setup

patch by Aleksey Yeschenko; reviewed by Jonathan Ellis for
CASSANDRA-6122
FD exposes a count of up/down machines over JMX
Patch by Sankalp Kohli, reviewed by brandonwilliams for CASSANDRA-6044
Trace index selection
patch by Lyuben Todorov and jbellis for CASSANDRA-6001
fix build
Update sstablesPerReadHistogram to use biased sampling
patch by jbellis; reviewed by yukim for CASSANDRA-6164
add live/tombstoned histogram counts to cfstats
patch by Mikhail Stepura; reviewed by jbellis for CASSANDRA-5889
add 6164 to NEWS