[IndexedDB] Allow more explicit control over transaction lifetimes (#34)

Imported from https://www.w3.org/Bugs/Public/show_bug.cgi?id=11528

Based on lots of developer feedback, in addition to the current automatic commit model it would be nice to support:

* explicitly committing, to avoid extra round-trips (known performance bottleneck)
* ways to defer committing to allow other async processing to occur within transaction scope

Both make the transaction model more complex, so we should tread carefully.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/IndexedDB/issues/34