VOL-2339: DHCP issue on second ONU, when two ONUs exist on same PON

- Process to_remove before to_add items in UpdateFlowsIncrementally
  method
- Sequentially add flows for a given pon, onu, uni inorder to avoid
  PON resource collisions. Used lock per key so that different
  subscribers do not have to wait for each other to add flows.
- Wait for any flow removes for the subscriber to complete before
  adding flows.

Change-Id: I095291b9a53fd0f19dc79f2b44923ec786a26d6e
diff --git a/go.mod b/go.mod
index 9d21cb0..85e32cd 100644
--- a/go.mod
+++ b/go.mod
@@ -3,6 +3,7 @@
 go 1.12
 
 require (
+	github.com/EagleChen/mapmutex v0.0.0-20180418073615-e1a5ae258d8d
 	github.com/cenkalti/backoff/v3 v3.1.1
 	github.com/gogo/protobuf v1.3.1
 	github.com/golang/protobuf v1.3.2