counters++ global shards 2.0 back port

CASSANDRA-6504 introduces a new type of shard - 'global' - to 2.1. To enable live upgrade from 2.0 to 2.1, it's necessary that 2.0 nodes are able to understand the new 'global' shards in the counter contexts.

2.0 nodes will not produce 'global' shards, but must contain the merge logic.

It isn't a trivial code change ("non-trivial code in a non-trivial part of the code"), hence this separate JIRA issue.