Avoid race condition with initialization

[IanDBird]

I found that we were exposed to a number of race conditions, caused by the following:

• The consuming app initialises the manager
• The manager internally kicks off an async task to load state from storage
• If the consuming app immediately calls our public API, this can happen before we have loaded our initial state
• The initial state will overwrite any modified state set by the consuming app.

This workaround ensures that all public interfaces are aware of the initialisation of the manager. If they occur before, they are suspended until we are ready.

[mindwalkr-ttd]

Suggestion: add a test to ensure the race condition is removed