Improve google drive performance and other features

I've finally found some time and rewrite old proposal from https://bugzilla.gnome.org/show_bug.cgi?id=771390 to change caching of google backend. This time I use the existing fundamental cache structures, which is less efficient, but possibly less error-prone and definitely easier for reviewing. I've also split related changes in multiple patches...

@debarshir please, please, please, can you take a look?