Add rbuf_existing statistic

Add `rbuf_existing` statistic - number of times a read buffer was
already attached to a connection and hence didn't need a new buffer
allocating.

Change-Id: Ic93b941b33d61787b9bbaf1d3ec30183de781785
Reviewed-on: http://review.couchbase.org/39594
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>