MB-11259: Move act_buf_size from value to extra field

Change-Id: I97d01300742505d5187f4d70785e0da02f14d94a
Reviewed-on: http://review.couchbase.org/38771
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>