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