[TaskManager] fetch more tasks to run than available capacity

resolves elastic#65552

Currently TaskManager attempts to claim exactly the number of tasks that
it has capacity for.

As an optimization, we're going to change to have it request more tasks than
it has capacity for.  This should improve latency, as when tasks complete,
there may be some of these excess tasks that can be started, as they are
already claimed (they still need to be marked running).

All the plumbing already handles getting more tasks than we asked for, we
were just never asking for more than we needed previously.

x-pack/plugins/task_manager/server/task_manager.ts

@@ -395,7 +395,7 @@ export async function claimAvailableTasks(
 
     try {
       const { docs, claimedTasks } = await claim({
-        size: availableWorkers,
+        size: availableWorkers * 2, // get more than we have capacity for
         claimOwnershipUntil: intervalFromNow('30s')!,
         claimTasksById,
       });