Fix keepalive for connected peers

[jm-clius]

This PR fixes #586

Description

It does so by introducing a simple (and optional) ping-like protocol for nim-waku that periodically sends an empty message to connected peers. This a temporary workaround until libp2p ping protocol has been implemented, as set out in vacp2p/nim-libp2p#567. Ping protocol is currently prioritised on the nim-libp2p side, but will likely not be finished before the planned testnet for nim-waku. Waku v2 dogfooding has also commenced and some keepalive mechanism is necessary to maintain stable fleet connections and bridge toy-chat messages to Discord.

How is this different from the libp2p ping protocol?

It is simpler in at least two ways:

• the keepalive messages do not carry any payload (as opposed to 32-byte ping messages), and
• it does not provide or expect any response. It is therefore simply a best effort to ensure there is minimal activity on open connections.

Limitations of this approach:

• it is temporary, though implemented in such a way that it can easily be replaced by ping protocol, once available.
• it is not interoperable with non-nim implemenations of Waku. Inter-client keepalive will likely only be available once ping protocol is implemented.

[staheri14]

Looks good!

[oskarth]

Hm, at what point does it make sense to just do the PING protocol? Any ETA on it? EDIT: I see issue was just assigned

[D4nte]

Are you using a content topic or are messages fully empty?

[jm-clius]

Hm, at what point does it make sense to just do the PING protocol? Any ETA on it? EDIT: I see issue was just assigned

Yeah, discussed with nim-libp2p and it's now prioritised on their side, but I'd like us to be unblocked to continue with testnet/dogfooding. This is an intermediate fix necessary to get the fleets and bridge in a working condition again.

[jm-clius]

Are you using a content topic or are messages fully empty?

Messages are completely empty protobufs, yes. It has no relation to either the PubSub/Waku domains, so contains no reference to content topics or similar. It simply keeps the connection between two switches open.