Wait for ActorSystem termination before returning from Dispose() of NeoSystem.

[jsolman]

Fixes #603
When using a pattern such as the following:

using (var store = new LevelDBStore(path))
using (var system = new NeoSystem(store)) {
    ...
}

The store needs to wait to be disposed until after the ActorSystem termination. This implements that.

Also, neo-cli was plagued by the same issue since it did the following on shutdown:

system.Dispose();
store.Dispose();

Fixes #607