Three-level nested transactions fails if root-level and first sub transaction don't execute any operations

[dfahlander]

See https://groups.google.com/forum/#!msg/dexiejs/b-uj775GO98/mCc6yTGPC0EJ
jsfiddle: http://jsfiddle.net/mt5mqjLL/2/

[dfahlander]

Found the issue. It's because Dexie's virtual sub transactions behave in the same bad manner as indexedDB transactions do: They fire their 'complete' event if no operation takes place in the same tick without waiting for micro-tasks to finish. Ever since Dexie.Promise started to have it's own 'micro-task' enging (Dexie.Promise._rootExec()), our sub transaction suffer from the same issue that make IndexedDB incompatible with ES6 Promise implementation.

I also found another issue but that will be filed separately.

Having a solution for both of them and will commit them soon.