mooclet: directly query reward counts, instead of 'posteriors'

[danoswaltCL]

requires a little research, the posteriors object on the policyParameters is how we've been getting the reward count, but it re-renders on the next "assign" call to mooclet instead of when a reward is actually logged, which has been awkward.

Instead we can try to get an immediate count by querying the Mooclet API directly (probably via the values table). Hopefully this can be accomplished without creating an endpoint in the mooclet code, but even that may be okay.