Redesign the wrapper's interiors for thread safety

There are a lot of locks needed now here, but none of them should be
contended. This is much better than potentially having RefCell's blow up
due to simultaneous mutable borrows, and the Arc is needed for the event
loop.

Cargo.toml

@@ -10,6 +10,8 @@ members = ["nih_plug_derive", "plugins/gain", "xtask"]
 
 [dependencies]
 nih_plug_derive = { path = "nih_plug_derive" }
+
+crossbeam = "0.8"
 lazy_static = "1.4"
 parking_lot = "0.12"
 serde = { version = "1.0", features = ["derive"] }