Serialize calls to the WebSocket Core FrameHandler

**Jetty version(s)**
10+

**Description**
Currently there is no protection against the `FrameHandler` being invoked concurrently by multiple threads.

In particular during an idleTimeout event, even if a FrameHandler is currently in `onOpen()` or `onMessage()` then the idleTimeout could occur and the `onClosed` notification would be called concurrently.

the jakarta websocket 2.1 specification states:
> In all cases, the implementation must not invoke an endpoint instance with more than one thread per peer at a time [WSC-5.1-2]. The implementation may not invoke the close method on an endpoint until after the open method has completed [WSC-5.1-3].
>
> This guarantees that a WebSocket endpoint instance is never called by more than one container thread at a time per peer [WSC-5.1-4].

Currently this is not the case as `onClosed()` can be called concurrently to `onOpen()` and `onMessage()`. But if we serialized the notifications to the `FrameHandler` in core it would solve this issue.

The `FrameHandler` implementations are currently not taking this into consideration, so are not protecting fields in a lock that could be modified concurrently by `onMessage` and `onClosed`.