393 fix cached dataset load

[mudinthewater]

Implements #393

Fixes an issue in getTopicsWithSessions interaction with persistence storage that caused a minimum of 15s delay before realtime sessions were stored.

old behavior: getTopicsWithSessions would check the dataset cache with getDatasets. The problem is that this is a race condition with MIO loads, as when it runs there are almost never any datasets loaded yet.

new behavior:

1. Added an async function that will check the getDatasets cache every 1 second for up to 15 seconds before returning an empty set. If a new dataset is found during this iteraction, will check for another dataset 1s later in case MIOS are still loading.
2. Added a new flag to getTopicsWithSessions which will toggle between the new behavior and the old behavior. This preserves the instant response on user interaction with the realtime session selection UI.