warn that ChannelHandlers should be instantiated within initializer closures (#590)
warn that ChannelHandlers should be instantiated within initializer closures (#590)

Motivation:

For (stateful) ChannelHandlers it's very important that the
ChannelHandlers get instantiated _within_ the bootstrap's initializer
closures (and not before that).

Modifications:

Add warnings about the fact that they should be instantiated within the
closures.

Result:

- better docs
- more correct user code
apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio