load generator: race condition on startup

[marta-lokhova]

Load generator does some state setup in scheduleLoadGeneration. Without the setup, we can't generate load correctly (for example, we can't generate load if we don't populate available accounts to choose from). However, it looks like core triggers load generation via generateLoad rather than scheduleLoadGeneration. Most of the time the first call to generateLoad is a no-op, since we compute the amount of load to generate based on elapsed time, but occasionally if the function takes a bit longer, we try to generate load and fail due to inadequate setup. We should refactor the startup code such that we perform all the necessary setup in the first call to generateLoad (and not in scheduleLoadGeneration, as it should only be in charge of rescheduling the callback)