Optionally disable caching in worker function

[twelch]

Caching of worker function results creates a lot of extra items in the DynamoDB task table. We don't need these partial cached results because worker results returned directly to the parent invoking function, and then cached as one big result.

Solution: add a disableCache: boolean flag to worker lambda invocation that disables caching of result.

[twelch]

Resolved via #342