DCP Backfill: Use size_t instead of uint32_t to record mem usage

DCP Backfill: Use size_t instead of uint32_t to record mem usage

We keep a record of DCP backfill memory usage to ensure that backfill
memory usage is bounded by a finite size (backfill buffer size).

We must consistently use size_t instead of uint32_t so that there is
no overflow error.

Change-Id: I01d0548b7d1cf3081b11bf96aec7868956c0bb6f
Reviewed-on: http://review.couchbase.org/77270
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: Build Bot <build@couchbase.com>