Package refresh and install/delete marks

[Artur Malabarba <bruce.connor.am@gmail.com>]

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> Async refreshing has grown on me too, but it still needs to implement
retaining the marks set by the user.

For the record, this is now done.
It was implemented by having tabulated-list only update the buffer, instead
of printing it anew.
As a bonus, the printing step is now blazing fast. So the small hang we get
after archives are downloaded is now barely noticeable.

Of note:
- if the user marks a package (for install/delete) and that package is gone
after the refresh, then the mark is gone too.
- if the user hits U while a refresh is in progress, the package menu will
wait until the refresh is done before actually marking the upgrades. This
is to avoid the scenario where the refresh could erase the install marks
and keep the delete marks.
- Hitting g will still revert the entire buffer and wipe all marks (this is
intentional).

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