Feature/fix memory leak

[DunetsNM]

There's a memory leak if source property sits on a long living object while target is on a short living object.
Simple fix is to wrap target properties in WeakReference

[DunetsNM]

used ConditionalWeakTable to improve add/remove performance. List of weak references still needed to iterate alive targets

[DunetsNM]

Performance test case:

1. Initial wiring: Create source with 50,000 immediate targets
2. Re-wiring: unplug an old target and add a new target, repeat 50,000 times

Results on my machine (Release)

• with HashSet: 0.2 sec for initial wiring and 0.3-0.4 sec for re-wiring
• with ConditionalWeakTable: 0.3-0.4 sec for initial wiring and 0.7-0.8 sec for re-wiring