Include Bitcask data file count in merge info messages. [JIRA: RIAK-2039]

It would be useful in cases of large merges in which the list of data files has become truncated to list the total number of files involved in the merge. If there's some way of estimating the total size of the data files removed during the merge operation that would be cool to see as well.

https://github.com/basho/bitcask/blob/bitcask-1.1.6/src/bitcask_merge_worker.erl#L199-L201
