[RFC] remove handler func support

[Stebalien]

1. It never actually worked (we always used the text match).
2. It made it impossible to accurately list protocols (required by the protocol).

reverts c9587f1

Alternatively (I'd actually prefer this) we should consider removing the "ls" command. That's not the job of this protocol (should use, e.g., an identify service).

[Stebalien]

@diasdavid thoughts? (FYI, by "it never worked", I mean "it never worked in go"). It looks like js-libp2p does support this but I'd still like to go one way or another. Either support listing supported protocols or don't support listing supported protocols.

[coveralls]

Coverage decreased (-1.2%) to 78.272% when pulling cd4d405 on feat/remove-handler-func into b8f1996 on master.

[Stebalien]

To make this clear, we have two conflicting features:

1. We have ls to list all supported protocols.
2. We allow registering dynamic protocol handlers.

• The first feature implies that we must know all supported protocols up-front.
• The second feature prevents us from knowing all protocols up-front.

There are three ways of fixing this:

1. Removing the dynamic protocol feature (this PR).
2. Remove the ls protocol.
3. Redefine the ls protocol to only list known protocols.

---

I filed this PR because the dynamic protocol feature is currently broken in go (and has never worked).

[Stebalien]

So... It turns out that the ls protocol is also broken in go.

1. NegotiateLazy doesn't implement it (it "lists" no protocols).
2. Negotiate writes back the protocol list but doesn't acknowledge the ls protocol first (that is, it just sends back the list of supported protocols without sending back <len>ls\n first.

😭