MB-7836: Give more granularity for minutes in cbstats

 disk_vbstate_snapshot (472 total)
    1us - 2us       : (  0.21%)   1
    2ms - 4ms       : ( 96.82%) 456 ##############
    8ms - 16ms      : ( 97.46%)   3
    16ms - 32ms     : ( 97.67%)   1
    32ms - 65ms     : ( 98.73%)   5 #
    65ms - 131ms    : ( 99.36%)   3
    131ms - 262ms   : ( 99.58%)   1
    4s - 8s         : ( 99.79%)   1
    1m:07s - 2m:14s : (100.00%)   1
    Avg             : (  154ms)

Change-Id: Ic29583d21cd0479280ef01dafc1a27254c2c1dff
Reviewed-on: http://review.couchbase.org/25073
Reviewed-by: Jin Lim <jin@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>