Move MeteredFlusher to its own thread

patch by Aleksey Yeschenko; reviewed by Sam Tunnicliffe for
CASSANDRA-8485
Merge branch 'cassandra-2.0' into cassandra-2.1

Conflicts:
	CHANGES.txt
	src/java/org/apache/cassandra/db/MeteredFlusher.java
	src/java/org/apache/cassandra/service/CassandraDaemon.java