Replace the deprecated MapMaker with CacheLoader

patch by Aleksey Yeschenko; reviewed by Jonathan Ellis for
CASSANDRA-6007
Merge branch 'cassandra-1.2' into cassandra-2.0

Conflicts:
	src/java/org/apache/cassandra/service/StorageProxy.java
Merge branch 'cassandra-2.0' into trunk
Fix CASSANDRA-6000 and CASSANDRA-5990 merges
Merge branch 'cassandra-2.0' into trunk