[IndexedDB] Support transaction timeouts (#43)

Two cases;
* abort a transaction if it hasn't been able to start before the timeout (e.g. other transactions are holding the lock)
* abort a transaction if it hasn't attempted to commit before the timeout (i.e. this transaction is too busy)

Chrome does the second automagically in the case of a hung renderer.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/IndexedDB/issues/43