Report filesystem read() counts per BGFetch operation

To improve our insight into how much work is being done during bg Fetches, it would be useful to record additional histograms:

• Count of read() calls per couchstore operation (i.e. per open() / close() pair).
• Number of read() calls issued per getMulti() call
• Number of read() calls issues per getMulti() call, per document - i.e. average reads for each document fetched.