campaignd: Allow clients to ask to not bump the downloads count

This allows clients to specify in the [request_campaign] body whether to
bump the downloads count for an add-on (e.g. when upgrading or
reinstalling a currently installed add-on). The default is 'true' for
backwards compatibility with older clients.

This change isn't advertised in the changelog on purpose.
addon/client: Add optional increase_downloads arg to download_addon()

This allows for requesting campaignd to not bump the downloads count for
an add-on that is being downloaded.
addon/mg: Don't bump download count when reinstalling/upgrading stuff