MB-11405 MB-11434 limit max writers to 8 (configurable)

In systems having 40+ cores, spawning 20 threads causes many
issues like heavy CPU and memory use.
Limit number of writers to 8, readers to 16
This change adds dynamic configurability to the shared thread pool.
It is now possible to tune a running system dynamically by setting limits
on the number of readers, writers, auxio and nonio threads using cbepctl
For example,
cbepctl set flush_param max_num_writers 1 (1 writer thread across buckets)
cbepctl set flush_param max_num_auxio 4
cbepctl set flush_param max_num_nonio 1
cbepctl set flush_param max_num_readers 1
Change-Id: Ie1314c2658792d6bd80d5f95fa57c94feb35b32b
Reviewed-on: http://review.couchbase.org/39475
Tested-by: Sundararaman Sridharan <sundar@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>