Problem: AttachedProtocols don't get registered (#7610)

I was investigating issues I am having with Whisper support. I've
enabled Whisper on a custom test network and inserted traces into
Whisper handler implementation (Network<T> and NetworkProtocolHandler
for Network<T>) and I noticed that the handler was never invoked.

After further research on this matter, I found out that
AttachedProtocol's register function does nothing:
https://github.com/paritytech/parity/blob/master/sync/src/api.rs#L172
but there was an implementation originally:
99075ad2#diff-5212acb6bcea60e9804ba7b50f6fe6ec and it did the actual
expected logic of registering the protocol in the NetworkService.

However, as of 16d84f8e#diff-5212acb6bcea60e9804ba7b50f6fe6ec ("finished
removing ipc") this implementation is gone and only the no-op function
is left.

Which leads me to a conclusion that in fact Whisper's handler never gets
registered in the service and therefore two nodes won't communicate
using it.

Solution: Resurrect original non-empty `AttachedProtocols.register`
implementation

Resolves #7566