limit simultaneous outgoing connections attempts to a reasonable top as an extra layer of protection
also shift the keep alive logic of the rpc handler to avoid needing to update it by hand. I think In rare cases this could make shutting down a connection a bit faster.
## Issue Addressed
#1483
## Proposed Changes
Upgrades the log to a critical if a listener fails. We are able to listen on many interfaces so a single instance is not critical. We should however gracefully shutdown the client if we have no listeners, although the client can still function solely on outgoing connections.
For now a critical is raised and I leave #1494 for more sophisticated handling of this.
This also updates discv5 to handle errors of binding to a UDP socket such that lighthouse is now able to handle them.
