Improve ManagedSelector task submission

**Jetty version(s)**
12+

**Description**
`ManagedSelector` submits a number of tasks to the `Executor`, namely `Accept`, `CreateEndPoint` and `DestroyEndPoint`.

We need to discuss whether, in particular for the first two, we must guarantee that they get executed and not queued in case all threads are blocked.

Start by locating ManagedSelector and tracing how the Accept, CreateEndPoint, and DestroyEndPoint tasks are submitted to the Executor. Investigate the behavior when all executor threads are blocked; completion requires a decided guarantee for whether the first two tasks must execute rather than remain queued, along with the corresponding change or rationale.

Written by the indexing model from the issue text.