Remove use of forClass method in push activation state machine implementation

[QuintinWillison]

See here.

Why do we need to do this?

We have a customer who is experiencing a crash, which I am pretty sure is coming from an exception thrown here.

See this internal thread.

We should probably work on #685 before trying to investigate this issue, as that code encapsulates this.

┆Issue is synchronized with this Jira Bug by Unito

[QuintinWillison]

The forClass and newInstance code came from the fix applied for #598 back in September 2020.

It violates every protection offered to us by the compiler. This must be improved. We should not be relying on such fundamental runtime exceptions to fail such critical code paths.

ActivationStateMachine.Event is abstract, yet defines no methods. Perhaps a solution would be to delegate responsibility for persistence of events to this class, in such a way that the compiler can police it before it ever gets to runtime.

Clearly also related: #546