Allow an explicit commit() for IndexedDB

Spec idea:
https://gist.github.com/inexorabletash/d55a6669a040e92e47c6

This allows developers to create really fast IndexedDB write-only transactions, although it's compatible with all transactions.

Appendix:
* writeonly transaction idea, alternate approach not preferred by Mozilla https://github.com/w3c/IndexedDB/issues/234
* Proposal to incorporate commit() with Web Lifecycle