fix(builder): introduce updateConstraintsCacheLock

[thedevbirb]

Introduces a updateConstraintsCacheLock on the builder to prevent racing conditions when subscribing to multiple builders for constrains. Even if shardmap has inner locks, the following edge case might occur in this code:

bolt-builder/builder/builder.go

Lines 396 to 408 in 5f66af8

	slotConstraints, _ := b.constraintsCache.Get(constraint.Message.Slot)
	if len(slotConstraints) == 0 {
	b.constraintsCache.Put(constraint.Message.Slot, decodedConstraints)
	continue
	}
	for hash := range decodedConstraints {
	slotConstraints[hash] = decodedConstraints[hash]
	log.Debug(fmt.Sprintf("Adding constraint with hash %s to cache for slot %d", hash, constraint.Message.Slot))
	}
	b.constraintsCache.Put(constraint.Message.Slot, slotConstraints)

1. relay 1 submits constraints A,B,C
2. builder gets constraints from cache, let's assume there is none of them
3. relay 2 submits constraints A,B
4. builder gets constraints from cache, they are still empty
5. builder saves constraints A,B,C from relay 1 with Put
6. builder saves constraints, A,B from relay 2 with Put, overriding the constraints for the slots