Add a `Gossip` log `Level`

[TheBlueMatt]

At TRACE, our logs are still pretty readable, which is great, except for gossip messages, which are a huge flood. It'd be nice to add a GOSSIP-level to reduce noise at TRACE for those who want verbose logs when things Actually Happen. This would just need to switch the log level depending on the message type in PeerHandler and then for the gossip-generated ignore errors as well.

[tnull]

I had a look a this in #1145, hope this is what you had in mind.

Note that I left the logging events concerning connection management at the TRACE level, because I figured they may still be of interest (and are likely not cluttering the logs too much?).