Serge::DB::Cached::find_best_translation() is not optimized for the disabled reuse_uncertain mode

[iafan]

When reuse_uncertain mode is disabled, Serge::DB::Cached::find_best_translation() just detects that there are some translations for the string in the database and passes the call to Serge::DB::find_best_translation(). This method is slow, however, and it is run on each localization cycle for items that need to be translated but have multiple variants of translations in the database.

To optimize for this scenario, we can cache the number of translation variants for each source string in Serge::DB::Cached::preload_strings_for_lang() and then account for that number in Serge::DB::Cached::find_best_translation(), quitting early if reuse_uncertain is off, and there are multiple variants of the translation.