make isOpen a computed property (#136)
make isOpen a computed property (#136)

Motivation:

Reduce memory footprint of BaseSocket

Modifications:

- made isOpen a computed var from 'descriptor >= 0'
- BaseSocket init check precondition 'descriptor >= 0'

Result:

- isOpen no longer requires storage space
- close() sets descriptor to -1
- descriptor property changed from let to var
apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio