MB-12179: Add initial max buffer to the BackfillManager

The backfill manager will only allow up to 20MB to be backfilled
from disk without being sent over the wire to the consumer. This
change will help to prevent growing memory usage from backfills
when a vbucket has a large dataset.

Change-Id: Ie505f2a77d0f0fe56a1b63f9a57edc6c11a11780
Reviewed-on: http://review.couchbase.org/42319
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>