Behavior subject fixes

[halgari]

The guts of Rx's BehaviorSubject bottoms out at this

        private void Unsubscribe(IObserver<T> observer)
        {
            lock (_gate)
            {
                if (!_isDisposed)
                {
                    _observers = _observers.Remove(observer);
                }
            }
        }

So yeah, you better hope you don't have a lot of observers on the subject because when they dispose it's a O(n) scan. Replaced the guts of the reactive code in MnemonicDB with R3's variant which are much faster and use array pools, index based removal and a lot of similar features.

[Al12rs]

Seems good, @erri120 might know of any gotchas with R3

[erri120]

Do you have before/after benchmarks?