Connection gating: default gater implementations

[raulk]

Motivation

Now that #872 has landed, we have the abstractions and the hooks in the right places to actively intercept inbound and outbound connections at the following lifecycle stages:

• peer being dialled.
• address being dialled.
• connection being accepted.
• connection secured and authenticated.
• connection fully upgraded (or equivalent, i.e. we have a fully capable connection).

Next step is to implement gaters for common use cases.

Goals

• Implement gaters for common use cases.
  • blacklisting peers by peer ID.
  • rejecting connections based on current connection count.
  • rejecting connections based on desired in/out connection quotas.
  • functional connection gater where user can plug in specific functions for each intercept, so that folks don't have to implement entire interfaces when they only want to capture at a specific stage.
    • Look at MockConnGater in swarm, and make those tests use this new functional gater.
    • Look at localhostMockGater in the QUIC transport(https://github.com/libp2p/go-libp2p-quic-transport/blob/bc2b6e0a86d5f16433de71e5796d48d9c6f7c7f5/conn_test.go#L28) and make those tests use this. Add a test for InterceptSecured.
• Implement a composite gater so we can chain these together.

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This is a follow-up to #872.

[marten-seemann]

It looks like users have been successfully been using the connection gater without any default implementations. I'm not sure if there's a lot of value in providing implementations here.