Benchmark counter improvements (counters++)

Benchmark the difference in performance between CASSANDRA-6504 and trunk.

• Updating totally unrelated counters (different partitions)
• Updating the same counters a lot (same cells in the same partition)
• Different cells in the same few partitions (hot counter partition)

benchmark: https://github.com/apache/cassandra/tree/1218bcacba7edefaf56cf8440d0aea5794c89a1e (old counters)
compared to: https://github.com/apache/cassandra/tree/714c423360c36da2a2b365efaf9c5c4f623ed133 (new counters)

So far, the above changes should only affect the write path.