External Games: support non-synchronized start

[sei-bstein]

The current implementation of external game hosting requires that the game be set to require a synchronized start. This restriction is enforced at both API and client levels.

The key technical limitation here is that we don't currently maintain a notion of an "instance" of an external game - all players registered are assumed to be playing at the same time, so all challenge and game resources are deployed a single time and the game finalized at the end of the session. To accommodate non-sync-start, we need to partition an instance or occurrence of a game to segment registered players.

This feature could potentially be useful to non-external games as well. Allowing site maintainers to keep a single game card and it settings with multiple registration/execution periods could be beneficial for single-event oriented scenarios.