add NIO event loop as an argument for execute (swift-server#79)

* add NIO event loop as an argument for execute

* review fix: add to np-delegate method as well

* Resolve confict

* add missing linux test

* fix formatting

* missing self

* review fix: add event loop preference argument instead of eventloop

* formatting

* review fix: spelling

* fix compilation error

* review fixes: make preference argument not explicit and add precondition that EL must be part of ELG

remove unused degate and make other one public+rename (swift-server#61)