Add support for 2.1 global counter shards

patch by Aleksey Yeschenko; reviewed by Sylvain Lebresne for
CASSANDRA-6505
Merge branch 'cassandra-2.0' into trunk

Conflicts:
	src/java/org/apache/cassandra/db/CounterCell.java
	src/java/org/apache/cassandra/db/CounterUpdateCell.java
	src/java/org/apache/cassandra/db/compaction/LazilyCompactedRow.java
	src/java/org/apache/cassandra/db/compaction/ParallelCompactionIterable.java
	src/java/org/apache/cassandra/db/compaction/PrecompactedRow.java
	src/java/org/apache/cassandra/db/context/CounterContext.java
	src/java/org/apache/cassandra/io/sstable/AbstractSSTableSimpleWriter.java
	src/java/org/apache/cassandra/io/sstable/SSTableWriter.java
	test/unit/org/apache/cassandra/db/CounterColumnTest.java
	test/unit/org/apache/cassandra/db/CounterMutationTest.java
	test/unit/org/apache/cassandra/streaming/StreamingTransferTest.java