Switch from Guava Optionals to Java 8 Optionals

This was a surprisingly involved change. Some of the difficulties included
java.util.Optional purposely not being Serializable (so I had to move a
few Optionals in mapreduce classes to @Nullable) and having to add the Truth
Java8 extension library for assertion support.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=171863777

java/google/registry/flows/EppMetrics.java

@@ -86,9 +86,9 @@ public class EppMetrics {
     String eppStatusCode =
         metric.getStatus().isPresent() ? String.valueOf(metric.getStatus().get().code) : "";
     eppRequestsByRegistrar.increment(
-        metric.getCommandName().or(""), metric.getClientId().or(""), eppStatusCode);
+        metric.getCommandName().orElse(""), metric.getClientId().orElse(""), eppStatusCode);
     eppRequestsByTld.increment(
-        metric.getCommandName().or(""), metric.getTld().or(""), eppStatusCode);
+        metric.getCommandName().orElse(""), metric.getTld().orElse(""), eppStatusCode);
   }
 
   /** Records the server-side processing time for an EPP request. */
@@ -98,8 +98,14 @@ public class EppMetrics {
     long processingTime =
         metric.getEndTimestamp().getMillis() - metric.getStartTimestamp().getMillis();
     processingTimeByRegistrar.record(
-        processingTime, metric.getCommandName().or(""), metric.getClientId().or(""), eppStatusCode);
+        processingTime,
+        metric.getCommandName().orElse(""),
+        metric.getClientId().orElse(""),
+        eppStatusCode);
     processingTimeByTld.record(
-        processingTime, metric.getCommandName().or(""), metric.getTld().or(""), eppStatusCode);
+        processingTime,
+        metric.getCommandName().orElse(""),
+        metric.getTld().orElse(""),
+        eppStatusCode);
   }
 }