Improved Connection Manager

Nobody has claimed this yet.

The current connection manager is keeping our nodes from running out of memory but is breaking everything else pretty badly. Unfortunately, it's rather tricky to correctly use the current interface.

I'd *like* to do the following:

1. Introduce some *constants* to make relative weights clear. That is, predefine `const IMPORTANT = 100` etc.
2. Introduce stream tracking. By default, *any* open stream on a connection would keep the connection open. However, we'd provide a way to deactivate/activate streams. This way, the DHT can demote its streams and user applications won't get their connections cut when they're actually *using* them. Once we get multistream 2.0, we can probably move to a "open a stream if you're going to use it and close it when you're done with it" model but we're not there yet.
3. Introduce some concept of decaying weight. Each time we talk to the peer, we'd "refresh" the weight. Bitswap would likely do this when sending blocks to a peer.

Anyone have bandwidth for this? It'll take a bit of work to get it right but it should help immensely.

Start by locating the current connection-manager interface and tracing how DHT streams and Bitswap peer activity use it. Done means the design supports named relative-weight constants, activatable stream tracking, and decaying weights refreshed by peer activity without cutting active user connections.

Written by the indexing model from the issue text.