Provide explicit ways to prevent concurrency

[p.colbert]

@stefano.menci

Disabling many components is very annoying, and often impossible.

Especially when using 3rd-party components. Even if they took the same approach – highly unlikely, at this point – each dependency is going to have its own distinct global flag(s).

[p.colbert]

@stucork

Anvil can manage its own event system but it can’t stop the browser from executing JavaScript.
If you click a Button while a server call is going on, the Browser still raises that click event.
Likewise a user is free to hit the browser back button while a server call is going on (think HashRouting).

How do JS apps handle this problem? (Sorry, I came to Anvil precisely to avoid spending time getting into JS/browser guts.) Can we use the same approach? Perhaps wrapped in Python?