MB-12279: Incrementally spawn writer threads for buckets

This change is needed to mitigate the high bgfetch latency
observed in heavy Data-Greater-than-Memory scenarios because
having high number of writing threads slows down disk read
performance.
Also mitigates MB-11143 slowdown in single HDD case

NOTE: This may slow down disk persistence in fast SSDs by default
NOTE: cbepctl can still be used to dynamically tune writers at runtime

Change-Id: Iddf0d3094f38b66ba8c0e09d6d6a307d15b38d56
Reviewed-on: http://review.couchbase.org/42552
Tested-by: Sundararaman Sridharan <sundar@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>