Maximum outbound connections

[Stebalien]

Given @raulk's dialer refactor, it should be pretty easy to plug-in a connection rate-limiter that blocks dialing if we have too many open connections.

The tricky part here will be not degrading service. We'll have to kill connections to get out of the way of new ones but that's going to be tricky...

[raulk]

Yeah, this would be a Preparer component that rejects the dial outright based on some criteria. It's not clear to me from the title if the idea is to limit outbound connections to a max by assigning in/out quotas, or if this it's simpler and we just want to block new attempts when the conn manager is maxed out.

Regardless, we could easily make the component take a predicate over a Conn slice.

[Stebalien]

if this it's simpler and we just want to block new attempts when the conn manager is maxed out.

This is mostly what I'm suggesting. At the end of the day, we don't really care who started the connection (except in rare cases). It's just easier to control outbound connections.