The mutation_fragment_merger takes one additional parameter in its
constructor, that is a pointer to a combined_reader_statistics
used to collect various statistics.
The histogram is populated with data while the merger consumes
batches from the producer and merges them into seperate mutation
fragments. The size of the batch, that represents the number of streams
the mutation fragment originates from, is used as a key in the
historgam and its corresponding value is increased by one.
