MB-6983: binary and decimal prefixes

extension of truncateTo3Digits is necessary because
we can get 1000K instead of 1M in some cases

Change-Id: Ib747e076e47d6e5f6b87deadd41ad26c247abfc7
Reviewed-on: http://review.couchbase.org/30214
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>