RCBC-137 Allow to select new IO engines: select and iocp

Change-Id: Id0fed509ceef701891ece45910c31de6872e2501
Reviewed-on: http://review.couchbase.org/28349
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>