thread safety checks

[ab9rf]

The main thing this PR does is add some code to monitor for attempts to acquire a CoreSuspender from the DF render thread, which is impermissible and will lead to deadlock. It currently only uses an assert for this (which means it's only operational in a debug build).

It also removes some dead code related to threading and fixes a couple of semantic errors:

• Core::Shutdown attempts to unlock the main core lock, but cannot do so because it always runs on the render thread while the main core lock is held by the simulation thread. This is replaced by an atomic flag shutdown which Core::Shutdown sets and which Core::Update checks.
• Plugin unloading would attempt to acquire a CoreSuspender to save plugin site and world data. Plugin unload during shutdown cannot acquire a CoreSuspender (because shutdown runs on the render thread), so unload now skips saving plugin site and world data during shutdown. This is harmless because site and world data cannot be saved during shutdown anyway.