Handle non-incremental Dropbox-like updates

This change adds handling for detecting remote changes when doing
Dropbox-like discovery. In this case, when we have a non-incremental
update, we need to do remote delete detection manually by comparing
against what we have in our local database.
7 jobs for 22-implement-dropbox-support in 12 minutes and 29 seconds (queued for 1 second)
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passed fedora-cmake #1360998366

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passed fedora-static #1360998365

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passed fedora-static-cmake #1360998367

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passed pri-build #1360998369

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