Maestro add local-memory support

[jun-he]

Maestro will provide a local memory support for the step runtime. It can be used to save some transient states in the memory.
Those are a few limits:

1. Those states are scoped to the step instance.
2. Those states won't be persisted and will be lost if the JVM is reboot
3. Those states are expected to be small (will have a size check)

It is useful for the case that the step runtime would like to put some temporary non-critical info there. Also, it is OK to lost the info.

[yingyiz-netflix]

I suppose this local-memory is thread safe? Specifically visibility of the shared state between threads

[jun-he]

Yep, each actor/step has its own separate memory. So it won't be shared between threads.