Added `WithScoreLedger` Bitswap option

[wolneykien]

First, I've separated the decision engine ledger on two parts: score and the wantlist. The wantlist still resides in the original ledger struct while sent/received byte accounting and scores are extracted to the new scoreledger struct managed by the original scoreWorker() logic. The accounting is integrated into the Engine via ScoreLedger interface making it possible to replace the original scoreWorker() with some other logic. The interface, however, doesn't allow a score logic to directly touch peer tags: the logic may decide about score values while tagging itself is still under control of Engine.

Then I've added new WithScoreLedger(decision.ScoreLedger) option in the bitswap package is the way to connect a custom ScoreLedger implementation to the decision engine. The Engine now has the corresponding UseScoreLedger(ScoreLedger) method.

The ScoreLedger and ScorePeerFunc types are exposed from the internal decision package to the public one.

New TestWithScoreLedger test is added. The test checks for start and stop of the testing score ledger implementation that is specified via WithScoreLedger option.

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[wolneykien]

The related discussion is here: https://discuss.ipfs.io/t/how-to-extend-a-bitswap-strategy/7960/3 . And a plugin type that injects ScoreLedgerss is here https://github.com/ipfs/go-ipfs/pull/7617 .

[wolneykien]

Ping?

[wolneykien]

Thanks! What about the related https://github.com/ipfs/go-ipfs/pull/7617 ?

[dirkmc]

I'll ping the go-ipfs folks