Consolidate all Set parameter parsing

Currently, we have two different ways to parse a "set" parameter:
key=value1&key=value2&key=value3...
and
keys=value1,value2,value3

This is error prone for several reasons:
- different parts of the code must be "synchronized" to use the same style (the
  place that creates the request, and the place that parses the request)
- for the key=value1&key=value2, we often use the same key name for the single
  value and the set value. This can result in subtle bugs where part of the
  code will successfully read the key assuming there's only one key (and will
  get the first key=value1, ignoring the rest)

Here we transition everything to the keys=value1,value2,value3 method. This one
was chosen because:
- it's shorter
- it's more intuitive for users
- the key name is plural, differentiating it from the singular key=value that
  other requests might need

-----------------------------------

To make sure there are not "transition issues", we will continue to support
(with warnings) the key=value1&key=value2 parameter parsing until we're sure we
haven't forgotten to update any part of the code.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=198810681

java/google/registry/cron/CronModule.java

@@ -30,45 +30,53 @@ import javax.servlet.http.HttpServletRequest;
 @Module
 public final class CronModule {
 
+  public static final String ENDPOINT_PARAM = "endpoint";
+  public static final String QUEUE_PARAM = "queue";
+  public static final String FOR_EACH_REAL_TLD_PARAM = "forEachRealTld";
+  public static final String FOR_EACH_TEST_TLD_PARAM = "forEachTestTld";
+  public static final String RUN_IN_EMPTY_PARAM = "runInEmpty";
+  public static final String EXCLUDE_PARAM = "exclude";
+  public static final String JITTER_SECONDS_PARAM = "jitterSeconds";
+
   @Provides
-  @Parameter("endpoint")
+  @Parameter(ENDPOINT_PARAM)
   static String provideEndpoint(HttpServletRequest req) {
-    return extractRequiredParameter(req, "endpoint");
+    return extractRequiredParameter(req, ENDPOINT_PARAM);
   }
 
   @Provides
-  @Parameter("exclude")
+  @Parameter(EXCLUDE_PARAM)
   static ImmutableSet<String> provideExcludes(HttpServletRequest req) {
-    return extractSetOfParameters(req, "exclude");
+    return extractSetOfParameters(req, EXCLUDE_PARAM);
   }
 
   @Provides
-  @Parameter("queue")
+  @Parameter(QUEUE_PARAM)
   static String provideQueue(HttpServletRequest req) {
-    return extractRequiredParameter(req, "queue");
+    return extractRequiredParameter(req, QUEUE_PARAM);
   }
 
   @Provides
-  @Parameter("runInEmpty")
+  @Parameter(RUN_IN_EMPTY_PARAM)
   static boolean provideRunInEmpty(HttpServletRequest req) {
-    return extractBooleanParameter(req, "runInEmpty");
+    return extractBooleanParameter(req, RUN_IN_EMPTY_PARAM);
   }
 
   @Provides
-  @Parameter("forEachRealTld")
+  @Parameter(FOR_EACH_REAL_TLD_PARAM)
   static boolean provideForEachRealTld(HttpServletRequest req) {
-    return extractBooleanParameter(req, "forEachRealTld");
+    return extractBooleanParameter(req, FOR_EACH_REAL_TLD_PARAM);
   }
 
   @Provides
-  @Parameter("forEachTestTld")
+  @Parameter(FOR_EACH_TEST_TLD_PARAM)
   static boolean provideForEachTestTld(HttpServletRequest req) {
-    return extractBooleanParameter(req, "forEachTestTld");
+    return extractBooleanParameter(req, FOR_EACH_TEST_TLD_PARAM);
   }
 
   @Provides
-  @Parameter("jitterSeconds")
+  @Parameter(JITTER_SECONDS_PARAM)
   static Optional<Integer> provideJitterSeconds(HttpServletRequest req) {
-    return extractOptionalIntParameter(req, "jitterSeconds");
+    return extractOptionalIntParameter(req, JITTER_SECONDS_PARAM);
   }
 }