CBD-1148-prereq: Replace IOManager => ExecutorPool

As we will be moving Non-IO tasks into the common
shared thread pool, we should not call it an IO
manager anymore. Hence discontinuing it in favor of
a generic ExecutorPool name

Change-Id: I852a825577b9363c7b91796d40bd044652eb75ca
Reviewed-on: http://review.couchbase.org/30640
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>