Recent updates to skicka: local metadata cache

[Matt Pharr]

First and foremost, we have a mailing list now; this would have been good to have had earlier for discussion of usage of the system and more open-ended discussion of new features than is possible via the github issues tracker (which is still the best place for bug reports.)

Second, a major improvement has just landed in the github repository: skicka now maintains a local cache of information about the files and folders stored on Drive. This makes startup much faster--commands like 'skicka du /' now take just a second or two to run, even with large numbers of files and folders stored on Drive.  (Whereas before, this could take many minutes, as it would download the information about all of the files on Drive each time.)

This change benefits pretty much all skicka commands--uploads and downloads are much faster at figuring out which files need to be uploaded/downloaded, etc.

I've spent quite a bit of time checking over the code and running tests, but if you do try this, please keep an eye out for any surprises.

Thanks,

Matt