gracefully abort fasttask on worker timeout (#263)

## Overview
This PR enables graceful aborts (rather than panics) when a fasttask times out waiting for worker availability.

## Test Plan
Tested locally.

## Rollout Plan (if applicable)
This can be rolled out along with any other changes.

## Upstream Changes
Should this change be upstreamed to OSS (flyteorg/flyte)? If so, please check this box for auditing. Note, this is the responsibility of each developer. See [this guide](https://unionai.atlassian.net/wiki/spaces/ENG/pages/447610883/Flyte+-+Union+Cloud+Development+Runbook/#When-are-versions-updated%3F).
- [ ] To be upstreamed

## Jira Issue
https://unionai.atlassian.net/browse/EXO-103

## Checklist
* [ ] Added tests
* [ ] Ran a deploy dry run and shared the terraform plan
* [ ] Added logging and metrics
* [ ] Updated [dashboards](https://unionai.grafana.net/dashboards) and [alerts](https://unionai.grafana.net/alerting/list)
* [ ] Updated documentation

fasttask/README.md

@@ -13,5 +13,7 @@ FastTask is a Flyte plugin to execute tasks quickly using a persistent, external
     export FLYTE_AWS_ACCESS_KEY_ID=minio
     export FLYTE_AWS_SECRET_ACCESS_KEY=miniostorage
     export FLYTE_AWS_ENDPOINT=http://localhost:30084
+
+    ./target/debug/worker bridge --queue-id=bar --fast-register-dir-override /tmp/fasttask-test
 ## build fast task worker image
     docker build -t hamersaw/fasttask:<tag> -f Dockerfile .

fasttask/plugin/service.go

@@ -275,14 +275,15 @@ func (f *FastTaskService) Cleanup(ctx context.Context, taskID, queueID, workerID
 	f.queuesLock.RLock()
 	defer f.queuesLock.RUnlock()
 
-	queue := f.queues[queueID]
-	queue.lock.RLock()
-	defer queue.lock.RUnlock()
-
-	if worker, exists := queue.workers[workerID]; exists {
-		worker.responseChan <- &pb.HeartbeatResponse{
-			TaskId:    taskID,
-			Operation: pb.HeartbeatResponse_DELETE,
+	if queue, exists := f.queues[queueID]; exists {
+		queue.lock.RLock()
+		defer queue.lock.RUnlock()
+
+		if worker, exists := queue.workers[workerID]; exists {
+			worker.responseChan <- &pb.HeartbeatResponse{
+				TaskId:    taskID,
+				Operation: pb.HeartbeatResponse_DELETE,
+			}
 		}
 	}