Fix bugs in load test action so that it actually works

This adds XSRF tokens so that the epptool request succeeds, adds better logging for debugging load test requests, adds better validation, and improves documentation.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=136885117

java/google/registry/loadtest/BUILD

@@ -21,6 +21,7 @@ java_library(
         "//third_party/java/servlet/servlet_api",
         "//java/google/registry/config",
         "//java/google/registry/request",
+        "//java/google/registry/security",
         "//java/google/registry/util",
     ],
 )