Use db.ListOptions directly instead of Paginator interface to make it easier to use and fix performance of /pulls and /issues (#29990)
This PR uses `db.ListOptions` instead of `Paginor` to make the code
simpler.
And it also fixed the performance problem when viewing /pulls or
/issues. Before the counting in fact will also do the search.
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Co-authored-by: Jason Song <i@wolfogre.com>
Co-authored-by: silverwind <me@silverwind.io>
Lunny Xiao