MB-9178: set IO thread count to 75% of # cpu cores

Number of IO threads in the shared thread pool is now set
to 75% of the number of CPU cores with 4 being min value
If maxThreads value is set to a non-zero value it will still
continue to override the default setting.

Change-Id: I5960b6c6706923571920bc8d0090ce8b8dd0fb63
Reviewed-on: http://review.couchbase.org/34199
Tested-by: Sundararaman Sridharan <sundar.sridharan@gmail.com>
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>