Fix an NPE that happens when DHCP server is not discovered yet

Also address the following issues:
- HOST_MOVED can include both movement and IP update. We should update server info when we see a HOST_MOVED event.
- Over 2k lines in this class
- Fix typo in extractClientId method name

Change-Id: I238027e650fb6b15c29b363caa09539170bdd3a4
diff --git a/apps/dhcprelay/src/main/java/org/onosproject/dhcprelay/Dhcp4HandlerImpl.java b/apps/dhcprelay/src/main/java/org/onosproject/dhcprelay/Dhcp4HandlerImpl.java
index 7999466..ece4323 100644
--- a/apps/dhcprelay/src/main/java/org/onosproject/dhcprelay/Dhcp4HandlerImpl.java
+++ b/apps/dhcprelay/src/main/java/org/onosproject/dhcprelay/Dhcp4HandlerImpl.java
@@ -1593,6 +1593,7 @@
             switch (event.type()) {
                 case HOST_ADDED:
                 case HOST_UPDATED:
+                case HOST_MOVED:
                     log.trace("Scheduled host event {}", event);
                     hostEventExecutor.execute(() -> hostUpdated(event.subject()));
                     break;