MB-12576: Limit writer threads to 4

Having more than 4 writers increases bgfetch latencies in DGM
This change selectively reverts 32a166c511d7b242433011a875402e1278300add
Change-Id: Icdb996622237747e759c52751f2c8e613c9ba262
Reviewed-on: http://review.couchbase.org/43060
Tested-by: Sundararaman Sridharan <sundar@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>