Move the first-party ChannelHandlers to NIOCore. (#1929)

Motivation:

NIO supplies a number of first-party ChannelHandlers that cover specific
use-cases. These aren't tied to the POSIX IO layer and so they should
come out.

Modifications:

- Move ChannelHandlers.swift to NIOCore
- Move NIOCloseOnErrorHandler.swift to NIOCore

Result:

Helper handlers in NIOCore

Author: Lukasa