Enable fairness on Kinesis Source's SynchronousQueue (#105)

This fixes a possible problem in which some Kinesis shards progress more
quickly than others. Each KCL shard processor has its own thread, and
each calls `queue.put()` on the synchronous queue.  It is possible that
by luck some shard/threads were having their `put()` accepted by a
consumer more quickly.

modules/kinesis/src/main/scala/com/snowplowanalytics/snowplow/sources/kinesis/KinesisSource.scala

@@ -42,11 +42,14 @@ object KinesisSource {
       }
     }
 
+  // We enable fairness on the `SynchronousQueue` to ensure all Kinesis shards are sourced at an equal rate.
+  private val synchronousQueueFairness: Boolean = true
+
   private def kinesisStream[F[_]: Async](
     config: KinesisSourceConfig,
     liveness: Ref[F, FiniteDuration]
   ): Stream[F, Stream[F, LowLevelEvents[Map[String, Checkpointable]]]] = {
-    val actionQueue = new SynchronousQueue[KCLAction]()
+    val actionQueue = new SynchronousQueue[KCLAction](synchronousQueueFairness)
     for {
       _ <- Stream.resource(KCLScheduler.populateQueue[F](config, actionQueue))
       events <- Stream.emit(pullFromQueueAndEmit(actionQueue, liveness).stream).repeat