fix (datafile management): Update polling behavior

[mjc1283]

Summary

• When autoUpdate is true, start the timer for the following request immediately, instead of waiting for the response to start the timer
• When the timeout fires before the response is received, wait for the response to complete and then start the next request immediately. This ensures there is at most 1 request in flight at any time.

Test plan

Added new unit tests. Manually tested.

Issues

https://optimizely.atlassian.net/browse/OASIS-4628

[coveralls]

Coverage remained the same at 97.58% when pulling 0c8cb48 on mcarroll/schedule-fix-simple into 27f4159 on master.