Add new function to get the checkpoints.

Previously there was only a function to get the latest checkpoint.  This
commit exposes a new function named Checkpoints which returns a slice of
checkpoints ordered by height for the active network or nil when
checkpoints are disabled.

checkpoints.go

@@ -100,6 +100,17 @@ func (b *BlockChain) checkpointData() *checkpointData {
 	return nil
 }
 
+// Checkpoints returns a slice of checkpoints (regardless of whether they are
+// already known).  When checkpoints are disabled or there are no checkpoints
+// for the active network, it will return nil.
+func (b *BlockChain) Checkpoints() []Checkpoint {
+	if b.noCheckpoints || b.checkpointData() == nil {
+		return nil
+	}
+
+	return b.checkpointData().checkpoints
+}
+
 // LatestCheckpoint returns the most recent checkpoint (regardless of whether it
 // is already known).  When checkpoints are disabled or there are no checkpoints
 // for the active network, it will return nil.