Add WireTap interface

[tzdybal]

WireTap interface can be used to access all messages send and received
by Bitswap. This can be used to implement advanced statistics/analysis
logic, which is beyond scope of Bitswap, but can be implemented as IPFS
plugin.

Some examples of potential applications:

• per CID bandwidth tracker (see: https://gitcoin.co/issue/PinataCloud/apollo/2/100023631)
• detailed per peer stats
• intrusion detection system (IDS) implementation

I tried to implement the entire "tapping" mechanism in the least obstructing way.
Because entire messages are passed for external processing, range of possible applications is enormous.

I created PR (without prior issue), because change is small and a commit is worth a thousand words ;)

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[dirkmc]

Hello, this PR looks good 👍

Could you please create a very simple test that just sends and receives a few messages between two nodes and verifies that MessageSent and MessageReceived are called correctly.

You can probably copy part of TestBasicBitswap

[tzdybal]

@dirkmc thanks for pointing me to TestBasicBitswap - it was great starting point.
I created a simple test showing that messages are wiretapped correctly.

[dirkmc]

Looks great, thanks! 👍

[aschmahmann]

It looks like this PR's tests are causing data races (i.e. go test -race fails on TestWireTap). @dirkmc @tzdybal do you mind taking a look?

[Stebalien]

It looks like the tests never ran. I wonder if CircleCI doesn't run for users without commit access?

[Stebalien]

Fixed in #446.