MB-7500: Add averages to cbstats histograms

Below is an example of the what the new timing stats
will look like. Note the new "Avg" field.

rw:fsWriteTime (522402 total)
   2us - 4us     : ( 15.75%)  82276 ##############
   4us - 8us     : ( 47.23%) 164469 #############################
   8us - 16us    : ( 62.28%)  78614 ############
   16us - 32us   : ( 80.93%)  97422 ################
   32us - 64us   : ( 88.40%)  38999 #####
   64us - 128us  : ( 91.92%)  18389 ##
   128us - 256us : ( 94.04%)  11077 #
   256us - 512us : ( 99.70%)  29589 ####
   512us - 1ms   : ( 99.93%)   1178
   1ms - 2ms     : ( 99.98%)    276
   2ms - 4ms     : ( 99.99%)     40
   4ms - 8ms     : ( 99.99%)     14
   8ms - 16ms    : ( 99.99%)     30
   16ms - 32ms   : (100.00%)     25
   32ms - 65ms   : (100.00%)      2
   65ms - 131ms  : (100.00%)      1
   131ms - 262ms : (100.00%)      1
   Avg           : (   31us)

Change-Id: Ic3ec69fbac450dd49a4411208e6f634c839364e0
Reviewed-on: http://review.couchbase.org/23757
Reviewed-by: Jin Lim <jin@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>