ConcurrentMap size calculation is wrong?

[wxing1292]

Please see the followings:

1. Remove decrement the counter with no checking of whether key exists or not:
   https://github.com/uber/cherami-server/blob/master/common/concurrentmap.go#L173

2. Put increase the counter with no checking of whether key exists or not:
   https://github.com/uber/cherami-server/blob/master/common/concurrentmap.go#L148