[Backport] Disable the disk write queue size cap for replication backoff.
[Backport] Disable the disk write queue size cap for replication backoff.

The disk write queue size cap for replication backoff was determined in
the very early versions of Couchbase Server that were deployed in
small-to-medium sized cluster with spining disks.

In our recent benchmark results, it turns out that disabling or setting
the disk write queue cap to a large value shows much better performance
behaviors in large-scale clusters with SSDs. As Couchbase Server is
deployed in such environments more and more, this change disables the
write queue size cap, but still supports a replication backoff by
checking the memory usage.

Note that the disk write queue size cap is still configurable at runtime.

Change-Id: Iedf711ad1c3bef61ca954f83f802b4a647b9ec88
Reviewed-on: http://review.couchbase.org/46367
Reviewed-by: Michael Wiederhold <a17fed27eaa842282862ff7c1b9c8395a26ac320@couchbase.com>
Tested-by: Chiyoung Seo <fcffc23db27b19ee4ebba46695ee6b50e09216a2@couchbase.com>
Reviewed-on: http://review.couchbase.org/46614
Reviewed-by: abhinav dangeti <d73ef2279de6577f7bf081737ee47554bbb7dc82@couchbase.com>

sriganes/ep-engine,daverigby/kv_engine,daverigby/ep-engine,hisundar/ep-engine,owendCB/ep-engine,couchbase/ep-engine,membase/ep-engine,membase/ep-engine,daverigby/kv_engine,daverigby/kv_engine,teligent-ru/ep-engine,daverigby/ep-engine,couchbase/ep-engine,membase/ep-engine,teligent-ru/ep-engine,daverigby/ep-engine,hisundar/ep-engine,membase/ep-engine,couchbase/ep-engine,hisundar/ep-engine,hisundar/ep-engine,teligent-ru/ep-engine,sriganes/ep-engine,sriganes/ep-engine,couchbase/ep-engine,teligent-ru/ep-engine,owendCB/ep-engine,sriganes/ep-engine,daverigby/kv_engine,owendCB/ep-engine,daverigby/ep-engine,owendCB/ep-engine