*: make IfWatcher::new synchronous

[elenaf9]

Make IfWatcher::new synchronous. The only reason why it is currently async is because the linux implementation executed a AddressGetRequest in new to obtain the initial list of ip addresses. By chaining this stream together with the existing message stream we can make linux::IfWatcher::new sync and instead move the polling into the Stream impl.
Motivation for this change is that IfWatcher::new being async currently causes some complexity in rust-libp2p. Concretely, it forces to wrap the IfWatcher in an enum [1] and first poll the future returned by IfWatcher::new [2] before we can poll the IfWatcher itself.

• [1] https://github.com/libp2p/rust-libp2p/blob/3da8b423c282383b7c55b2eda7050bd991a2813b/transports/tcp/src/lib.rs#L608-L611
• [2] https://github.com/libp2p/rust-libp2p/blob/3da8b423c282383b7c55b2eda7050bd991a2813b/transports/tcp/src/lib.rs#L747-L764

[mxinden]

👍 Cool. This looks good to me.

[mxinden]

@elenaf9 could you resolve the merge conflicts?

[mxinden]

Oh, missing changelog entry.

[elenaf9]

Not sure how to describe this better.

[mxinden]

Looks good to me.

[mxinden]

Looks good to me.

[mxinden]

@elenaf9 should I cut a release already, or do you expect other changes in the near future?

[elenaf9]

@elenaf9 should I cut a release already, or do you expect other changes in the near future?

I am still looking into simplifying the IfWatcher integration in rust-libp2p. There may still be some minor changes coming. I will ping you once I am through and we can cut a release. Thanks!