Right now, the transaction timeout parameter is an unsigned long with no time equivalent to infinity and no way (other than omission) to use the browsers default.  Unsigned long is also kind of an odd type to use in a JavaScript API.

Maybe this should be a double instead.  Positive infinity could mean "no timeout".  0 and/or negative could mean browser default and/or throw an exception.  (I don't care much either way.)

Note that Geolocation uses a double for their timeout parameter.  In that case, 0 means to timeout immediately if there's no cached value.  In IndexedDB, it seems as though the analog would be like a try lock (i.e. run the transaction if we can without waiting at all), but I'm not sure that has much benefit and it would be adding non-deterministic surface area which seems bad.