MB-11405 Spawn at least 10 global threads

Make the number of threads per task group configurable both
at start time as well as runtime for performance tuning
Change-Id: I4dcbb4de79cfa39375602e498f321d596145b81d
Reviewed-on: http://review.couchbase.org/40084
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>