bp:MB-100 replace hardcoded MiB with define

This is clean back-port

Change-Id: I58cbc90d3dcae8f47b14087e41347e1c917e150c
Orig-Reviewed-on: http://review.couchbase.org/29460
Reviewed-on: http://review.couchbase.org/29514
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>