Fix rate limit publisher nil pointer deref.

[nishkrishnan]

Was using the wrong client as input to the executor service so as part of this change I moved the logic inside the atlantis github client for consistency.

Additionally, I modified the ticker logic to solve the following:

• Old logic would potentially run parallel go routines if the routine took longer than the ticker duration (not super useful)
• panics would crash the entire server; this is now isolated to a given job on the executor service
• properly making use of context cancelling to shut down child threads.