MB-11067: Add stats for # buffers loaned / allocated.

To allow us to measure the effect of the buffer change in MB-11067,
add two pairs of new stats:

 * {r,w}bufs_allocated: Number of times a {read,write} buffer was
                        fulfilled by malloc()ing a new buffer.

 * {r,w}bufs_loaned: Number of times a read buffer was fulfilled by
                     loaning out the worker threads' buffer.

Change-Id: If373c40491d46627468410e22d2fb0d8ae2cd53b
Reviewed-on: http://review.couchbase.org/38556
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>