worker: Configurable timeout for RequestJob

This is backwards compatible, as long as the timeout is 0 (never
timeout), which is the default.

In case of the dbjobqueue the underlying timeout is due to
context.Canceled, context.DeadlineExceeded, or net.Error with Timeout()
true. For the fsjobqueue only the first two are considered.

internal/jobqueue/jobqueue.go

@@ -74,7 +74,8 @@ type JobQueue interface {
 }
 
 var (
-	ErrNotExist   = errors.New("job does not exist")
-	ErrNotRunning = errors.New("job is not running")
-	ErrCanceled   = errors.New("job ws canceled")
+	ErrNotExist       = errors.New("job does not exist")
+	ErrNotRunning     = errors.New("job is not running")
+	ErrCanceled       = errors.New("job ws canceled")
+	ErrDequeueTimeout = errors.New("dequeue context timed out or was canceled")
 )