(#2144) - better errors for leveldb
(#44) - Fix worker tests for alt backends
(#1992) - fix attachment uploads in Android 4.x
(#2132) - disable local Web Workers in IE
(#1661) - use task queue for IDB for IE

Adds a task queue for all create/destroy
operations on IDB databases, because apparently
IE 10 and 11 can't function correctly if there's
any overlap between any destroys/creates (even
from different databases).

Also apparently you can't have two databases
with the same name open at the same time,
so we have to cache this.