Add optional rate-limit on incoming pre-auth connections

[t-bast]

Peers that open a connection but never complete the BOLT 8 handshake consume resources on our node (memory, file descriptors and CPU).

We now bound how many of those we're willing to keep around, with an eviction mechanism that lets honest peers eventually get connected.

Note that this only affects incoming connections: outgoing connections are not rate-limited, which means that peers with whom we have channels cannot be negatively affected by this mechanism since we'll actively reconnect to them.

This isn't a foolproof DDoS protection at all however: attackers that spam connection attempts can still severely delay or prevent honest new peers from connecting. It should be coupled with a DDoS protection layer at the cloud provider layer, where it is easier to do correctly and more efficient.

This mechanism can be disabled entirely by setting eclair.peer-connection.max-pending-incoming-connections = 0 in eclair.conf. The default values are high enough to have no impact on most deployments.

Disclaimer: this code was created with the help of Claude Opus 5, which was a very good rubber duck, reviewer and wrote most of the unit tests.

[t-bast]

Trivial rebase to fix the merge conflict (on release notes).

[pm47]

LGTM.

Note that in a cluster setup, the max-pending-incoming-connections value applies independently for each front node.