Single node GC

[joente]

Describe the bug
In single node, things may "hang" inside the garbage collector.

To Reproduce
To reproduce, you need to have an assigned thing. An example is this following:

new_type("A");
new_type("B");
set_type("A", {b: 'B?'});
set_type("B", {a: 'A?'});
mod_type("A", "rel", "b", "a");

// Create A with B
.A = A{};
.A.b = B{};

// Now remove A
.del('A');

After GC has run, the collection should be clean

Expected behavior
Things must be garbage collected.

Context
This only happens when running a single node. In a multi-node set-up, the garbage collector works as expected.

[joente]

Solved in v1.8.4-alpha1, see b3d30bd