lwcapi: batch sub messages (#1606)

When setting up a new subscription, batch the subscription
metadata messages sent to the queue. Otherwise if there
is a large set it they could get written quickly enough to
exceed the queue size.

atlas-lwcapi/src/main/scala/com/netflix/atlas/lwcapi/SubscribeApi.scala

@@ -195,11 +195,12 @@ class SubscribeApi(
 
         // Add any new expressions
         val (queue, addedSubs) = sm.subscribe(streamId, splits)
-        addedSubs.foreach { sub =>
+        val subMessages = addedSubs.map { sub =>
           val meta = sub.metadata
           val exprInfo = LwcDataExpr(meta.id, meta.expression, meta.frequency)
-          queue.offer(Seq(LwcSubscription(expr.expression, List(exprInfo))))
+          LwcSubscription(expr.expression, List(exprInfo))
         }
+        queue.offer(subMessages)
 
         // Add expression ids in use by this split
         subIdsBuilder ++= splits.map(_.metadata.id)