peer: launch persistent peer pruning in background goroutine

This PR is a follow up, to a [follow
up](lightningnetwork#7938) of an [initial
concurrency issue](lightningnetwork#7856)
fixed in the peer goroutine.

In lightningnetwork#7938, we noticed that the introduction of `p.startReady` can cause
`Disconnect` to block. This happens as `Disconnect` cannot be called
until `p.startReady` has been closed. `Disconnect` is also called from
`InboundPeerConnected` (the case of concurrent peers, so we need to
remove one of the connections) while the main server mutex is held. If
`p.Start` blocks for any reason, then this leads to the deadlock as: we
can't disconnect until we've finished starting, and we can't finish
starting as we need the disconnect caller to exit as it has the mutex.

In this commit, we now make the call to `prunePersistentPeerConnection`
async. The call to `prunePersistentPeerConnection` eventually wants to
grab the server mutex, which triggers the circular waiting scenario
above.

The main learning here is that no calls to the main server mutex path
can block from `p.Start`. This is more or less a stop gap to resolve the
issue initially introduced in v0.16.4. Assuming we want to move forward
with this fix, we should reexamine `p.startReady` all together, and also
revisit attempt to refactor this section of the code to eliminate the
mega mutex in the server in favor of a dedicated event loop.

peer/brontide.go

@@ -551,7 +551,7 @@ func (p *Brontide) Start() error {
 	}
 
 	if len(activeChans) == 0 {
-		p.cfg.PrunePersistentPeerConnection(p.cfg.PubKeyBytes)
+		go p.cfg.PrunePersistentPeerConnection(p.cfg.PubKeyBytes)
 	}
 
 	// Quickly check if we have any existing legacy channels with this