Whole decision batch optimization

[wxing1292]

Currently, decision task handling logic does not do optimization on cases such as

1. schedule a child workflow
2. cancel external workflow (pointing to the above child workflow)

the above can end up with: first trying to cancel the child workflow (which is yet been started), then schedule the child workflow since task processing is parallel.

similar case applies to:

1. child workflow creation & signal / cancellation
2. activity creation & cancellation
3. user timer creation & cancellation

we should have whole decision batch optimization logic handling this.