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/vendor/modules.txt b/vendor/modules.txt
index ea3c340..8fe9dde 100644
--- a/vendor/modules.txt
+++ b/vendor/modules.txt
@@ -1,5 +1,7 @@
 # github.com/DataDog/zstd v1.4.1
 github.com/DataDog/zstd
+# github.com/EagleChen/mapmutex v0.0.0-20180418073615-e1a5ae258d8d
+github.com/EagleChen/mapmutex
 # github.com/Shopify/sarama v1.23.1
 github.com/Shopify/sarama
 # github.com/armon/go-metrics v0.0.0-20190430140413-ec5e00d3c878