Support transaction timeouts

[inexorabletash]

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.

Imported from https://www.w3.org/2008/webapps/wiki/IndexedDatabaseFeatures

[kristofdegrave]

In the first case I would use a blocked callback like when you want to open a newer version of the database and have existing connections. This way you give the developer control on how he wants to act on this

[inexorabletash]

TPAC 2019 Web Apps Indexed DB triage notes:

We think this can be done in user-space, either with just timeouts and abort calls, and/or some of the proposals over in #34