Add sync & async options for master_call in RunnerClient and WheelClient

This pull req fixes #9324.

tl;dr: This adds a ``mode`` key to ``clear_load`` in ``master_call`` for both RunnerClient and WheelClient which can be set to ``sync`` or ``async``.

@thatch45 & @SmithSamuelM this change is not backward compatible for Halite because it changes the default to 'sync' for RunnerClient(). I figure this is the easier route because we have community and customers that are using WheelClient() in production (which defaults to 'sync').

FWIW, these changes pass the master_call integration tests from yesterday. (Not that those tests are exactly battle-hardened...)