Add getUserSharedBuffer() callback finalizer to fix memory leak

[cb1kenobi]

When getUserSharedBuffer() is called with a callback, the callback is added to an internal std::unsorted_map. As far as I can tell, the callback function ref is not GC'd until the env is closed or Node.js exits. We can tie the life of the callback to the life of the returned ArrayBuffer.

If we call getUserSharedBuffer() in a loop, it will consume memory until Node.js runs out of memory on the heap and crashes. Note that the loop needs to yield in order for GC to collect the unused returned shared buffer.

let consumedMemory = 0;
for (let i = 0; true; i++) {
	consumedMemory += 8;
	db.getUserSharedBuffer(`foo${i}`, new ArrayBuffer(8), {
		envKey: true,
		callback: () => {
			console.log('user shared buffer callback');
		}
	});
	await new Promise(resolve => setImmediate(resolve));
}

Before this fix, Node.js would crash after 37M iterations. After this fix, Node.js will not crash.

Further more, if we cap the iterations to 4,222,600 (arbitrary, non-magical number), the code above will have consumed 32.2MB of memory, however we can see the V8 function ref is not being released.

BEFORE
# keys: 4222600  Size: 32.2 MB   ####------------------------------------   457.3 MB / 467.1 MB / 4 GB

AFTER
# keys: 4222600  Size: 32.2 MB   ----------------------------------------    18.9 MB /  43.3 MB / 4 GB

[cb1kenobi]

This PR is misguided. Closing.