Rework Neighbor Cache to use Weak References

[ghzdude]

What

Refactors the neighbor cache to use a list of weak references instead of an array with direct references

Implementation Details

add NULL and INVALID weak references to mimic previous logic.
if INVALID, the neighbor should be recomputed, otherwise either the TE or null should be returned
It also needs to be checked that the neighbors are not null unexpectedly due to referent being lost somehow

Outcome

Theoretically, references to neighboring TEs should be GC'd properly now.