Add ability to throttle batchlog replay

patch by Aleksey Yeschenko; reviewed by Jonathan Ellis for
CASSANDRA-6550
Merge branch 'cassandra-1.2' into cassandra-2.0

Conflicts:
	NEWS.txt
	src/java/org/apache/cassandra/db/BatchlogManager.java
Merge branch 'cassandra-2.0' into trunk

Conflicts:
	NEWS.txt
	src/java/org/apache/cassandra/db/BatchlogManager.java