fix: Nil dereference while using SetSendDontHaves

This option is used by the benchmark to simulate the old bitswap
comportement.

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.

bitswap.go

@@ -107,7 +107,7 @@ func EngineBlockstoreWorkerCount(count int) Option {
 // This option is only used for testing.
 func SetSendDontHaves(send bool) Option {
 	return func(bs *Bitswap) {
-		bs.engine.SetSendDontHaves(send)
+		bs.engineSetSendDontHaves = send
 	}
 }
 
@@ -201,6 +201,7 @@ func New(parent context.Context, network bsnet.BitSwapNetwork,
 		provSearchDelay:         defaultProvSearchDelay,
 		rebroadcastDelay:        delay.Fixed(time.Minute),
 		engineBstoreWorkerCount: defaulEngineBlockstoreWorkerCount,
+		engineSetSendDontHaves:  true,
 	}
 
 	// apply functional options before starting and running bitswap
@@ -210,6 +211,7 @@ func New(parent context.Context, network bsnet.BitSwapNetwork,
 
 	// Set up decision engine
 	bs.engine = decision.NewEngine(bstore, bs.engineBstoreWorkerCount, network.ConnectionManager(), network.Self(), bs.engineScoreLedger)
+	bs.engine.SetSendDontHaves(bs.engineSetSendDontHaves)
 
 	bs.pqm.Startup()
 	network.SetDelegate(bs)
@@ -293,6 +295,11 @@ type Bitswap struct {
 
 	// the score ledger used by the decision engine
 	engineScoreLedger deciface.ScoreLedger
+
+	// indicates what to do when the engine receives a want-block for a block that
+	// is not in the blockstore. Either send DONT_HAVE or do nothing.
+	// This is used to simulate with old version of bitswap that were quiets.
+	engineSetSendDontHaves bool
 }
 
 type counters struct {