MemoryManager rework

[CedNaru]

Solve #508, #618, #566, #666 and #617

It's honestly hard to explain in details all the changes without turning it into an essay. Most of the changes and explanations were already written in #618.

Here's a recap of the main changes:

• The MemoryManager is no longer its own thread, instead it's called directly by the C++ code at the end of each frame.
• Bindings no longer keep JNI references to their JVM wrappers. Objects are directly stored as strong references in the JVM ObjectDB and RefCounted as weak references.
• The "binding phase" that was made in a delayed manner is now gone. Instead, it's replaced by a "dead object" phase. Each frame, the list of dead objects is sent to the JVM to remove them from the ObjectDB.
• Refcounted wrappers are no longer permanent, they die as soon as they are no longer used by the JVM, decrementing the counter in the process. The JVM is no longer the one with the final words on RefCounted wrappers' life.
• The only exception are RefCounted Scripts. The ping pong of weak/strong JNI ref was kept but moved from KoltinBinding to JvmInstance. The ping pong is also less frequent. References are transformed into strong ones immediately, but the switch to weak is delayed to the end of the frame.
• ForceGC was removed, it's not a reliable method to make sure the GC ran.
• DisplayLeaks was removed, Godot should be the only one responsible for such reports.
• Singleton management has been removed because no longer relevant in the new model.
• Static management has been replaced by a broader "cleanup callback" system. It can be used to close third party threads, for example.

[chippmann]

Very nice work!
Will follow up with proper testing later this week

[chippmann]

https://godot-kotl.in/en/stable/contribution/knowledge-base/memory-management/

Should probably also be updated