remove context from constructor, implement a proper Close method

[marten-seemann]

I wish we didn't have to make this change, but controlling shutdowns with a context simply doesn't work in Go: There's no way to know when a function listening on ctx.Done() has actually shut down. All you can hope for is that it eventually does.

A reliable way of implementing a clean shutdown is by having a Close method, which explicitly blocks until all go routines have finished.

Unfortunately, this is an API-breaking change, since we're 1. removing the ctx from the constructor, and (logically) we're adding a Close method that users of this library will now have to call explicitly.

The concrete reason why I'm sending this PR now is that this is blocking libp2p/go-libp2p#1168. In specific, we have a few tests there that modify manet.Private4. This global variable is accessed by the background loop in

go-libp2p-autonat/autonat.go

Line 183 in 3723bd5

if manet.IsPublicAddr(conn.RemoteMultiaddr()) &&

, creating a race condition.