Adding Drive as a storage backend experimentally, there is no way
of enumerating Drive files in a cheap way, needs to be fixed
once Drive supports range queries.

goexif: fix crash observed in the wild.

Reported by ajft@ajft.org

Change-Id: I4d4ddc998bb07f75d484341b8e423151d6c1aaa0

experimental drive.

adding upsetrt and etc.

adding remove.

Adding enumerate.

...

...

changes.

Drive should be primary.

changs ...

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Demonstrating how sharding may work on insertions to Drive.

Remove unnecessary comment.

Experimental index based on dir sharding.

Remove sharding.

Adding camliblob mimetype.