Make task executor lifecycled to platform readiness.

[wizjany]

Closes #2459.

Note that the platform list (now map) was/is not threadsafe, which in theory might be problematic if platforms throw ready/unready in multiple threads. Ready/unready should be fixed by #2571 if this is an issue.

[wizjany]

This is now tested on Fabric, Neoforge, and Bukkit; client and server.

Notably, the schematic tasks don't actually listen to interrupts, so a long-running schematic load in single player will actually run to completion when closing a world and loading back into it (or another). However, the running schematic task doesn't affect the new world as the actor/session/etc. is no longer in use. It also doesn't prevent server shutdowns as the executor is no longer waiting for task completion.

Looking at the original comments, it seems there was some concern about a timer thread, but it doesn't seem to be causing any issues. I verified that this change does actually fix the issue by commenting out the executor shutdown call and watching the server not shut down.