High CPU usage in AddAddrs

[Stebalien]

So, #22 gave us a really nice win in terms of memory usage but, unfortunately, given the insane number of addresses we're currently tracking, is now causing CPU problems.

Two solutions:

1. Limit the number of addresses we store per peer. Unfortunately, this could make it difficult to connect to some peers. Ideally, peers would sign their peer address records but that will require some work. Regardless, we should do this anyways.
2. Make this function faster. Unfortunately, given that @whyrusleeping has been dragging his feet on make Multiaddr a newtype of []bytes multiformats/go-multiaddr#63 and/or store Multiaddrs as strings and don't use interfaces multiformats/go-multiaddr#62, pretty much all reasonably efficient algorithms for merging multiaddr sets require copying.

[whyrusleeping]

Unfortunately, given that @whyrusleeping has been dragging his feet on...

Sheesh, fine. I'll go review those

[Stebalien]

This hotspot appears to have been fixed.