Allow IEnumerable instead of HashSet in server broadcasting + improve performance in SceneManager

[ghost]

In ServerManager.Broadcast(connections, ...); instead of HashSet<NetworkConnection> use IEnumerable<NetworkConnection> since it's more general & does not break anything.

Plus, in SceneManager, like 1779, you literally allocate an array just to iterate through it. In RebuildObservers(...) method also allow IEnumerable<NetworkObject> and in line mentioned above remove .ToArray() call.

[FirstGearGames]

Edited:
We are moving away from IEnumerable except in areas where enumerator allocations are unavoidable. Method signatures will be replaced with IList.

I can however optimize the SceneManager methods and a few more in there.

[FirstGearGames]

Improved where applicable. Available in 3.10.8R