util, finagle, twitter-server: Remove JavaSingleton Trait

Problem / Solution

The `JavaSingleton` trait in util is lightly used and doesn't provide
functionality in the way that it's intended to work. Let's remove
its single uses within Finagle and Twitter Server and encourage folks
to take a different approach for better Java compatibility.

JIRA Issues: CSL-9080

Differential Revision: https://phabricator.twitter.biz/D399947

CHANGELOG.rst

@@ -7,9 +7,18 @@ Note that ``PHAB_ID=#`` and ``RB_ID=#`` correspond to associated messages in com
 Unreleased
 ----------
 
+Runtime Behavior Changes
+~~~~~~~~~~~~~~~~~~~~~~~~
+
 * finagle-netty4: Change the 'connection_requests' metric to debug verbosity.
   ``PHAB_ID=D391289``
 
+Breaking API Changes
+~~~~~~~~~~~~~~~~~~~~
+
+* finagle-core: The `RetryPolicy` companion object is no longer a `JavaSingleton`.
+  ``PHAB_ID=D399947``
+
 19.11.0
 -------
 

finagle-core/src/main/scala/com/twitter/finagle/service/RetryPolicy.scala

@@ -8,14 +8,7 @@ import com.twitter.finagle.{
   TimeoutException,
   WriteException
 }
-import com.twitter.util.{
-  Duration,
-  JavaSingleton,
-  Return,
-  Throw,
-  Try,
-  TimeoutException => UtilTimeoutException
-}
+import com.twitter.util.{Duration, Return, Throw, Try, TimeoutException => UtilTimeoutException}
 import java.util.{concurrent => juc}
 import java.{util => ju}
 import scala.collection.JavaConverters._
@@ -155,7 +148,7 @@ abstract class SimpleRetryPolicy[A](i: Int)
   final val never = Duration.Top
 }
 
-object RetryPolicy extends JavaSingleton {
+object RetryPolicy {
 
   /**
    * An extractor for exceptions which are known to be safe to retry.