fix: Nil dereference while using SetSendDontHaves

[Jorropo]

This option is used by the benchmark to simulate the old bitswap comportement (and fixes the benchmark).

This follows the same refactoring idea as done in 47b99b1.

It was crashing since it was trying to access the sendDontHaves property of bs.engine but bs.engine is initialized right after the options are applied, not before.

Unlike 47b99b1, I've choosed to not make sendDontHaves part of engine's constructor because this is such an obscure option, useless in +99% of realworld scenario that it's not needed to confuse the engine contructor.

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[Jorropo]

/cc @dirkmc (no idea if you are interessed but you wrote 47b99b1 :))

[BigLep]

@coryschwartz : we're hoping you can take a quick look at this as it may be reviewable without go-bitswap context. If you think you need more, then leave it for @aschmahmann .

[coryschwartz]

Thanks, this looks good to me.

[aschmahmann]

Thanks, LGTM. Made a small suggestion on the documentation comments to make things a bit more explicit.

[Stebalien]

Failing tests are unrelated.