Sync ForgerCommitment and use it in coord

Previously the coordinator was erroneously using Slot.BatchesLen to determine
when anyone can forge.  The correct behaviour is implmenented with the boolean
flag `ForgerCommitment`, that is set to true only when there's a batch before
the deadline within a slot.

Delete Slot.BatchesLen, and the synchronization code of this value from the
Synchronizer, as this is not needed

common/bid.go

@@ -26,13 +26,14 @@ type BidCoordinator struct {
 
 // Slot contains relevant information of a slot
 type Slot struct {
-	SlotNum        int64
-	DefaultSlotBid *big.Int
-	StartBlock     int64
-	EndBlock       int64
-	BatchesLen     int
-	BidValue       *big.Int
-	BootCoord      bool
+	SlotNum          int64
+	DefaultSlotBid   *big.Int
+	StartBlock       int64
+	EndBlock         int64
+	ForgerCommitment bool
+	// BatchesLen       int
+	BidValue  *big.Int
+	BootCoord bool
 	// Bidder, Forer and URL correspond to the winner of the slot (which is
 	// not always the highest bidder).  These are the values of the
 	// coordinator that is able to forge exclusively before the deadline.