Return promises and remove callback arguments

[ali-ince]

This PR modifies existing API's to return Promises instead of the previous callback mechanism.

1. Driver.close() now returns Promise<void>.
2. Transaction.commit() and Transaction.rollback() now returns Promise<void> rather than the internal Result object which holds more and confusing information not applicable to the underlying commit/rollback operation (resolves Commit() return values do not have actions taken #260).
3. Internal Pool implementation now stores it's closed state and now is able to return errors if it's asked for a new connection (resolves Session and driver not closing; can still post queries @1.7.2 #442).
4. Refactored stub tests so that they run faster and are now compatible with latest promise returning functions.

Based on #445.