It can get candidates sufficiently fast with the current logic. Remove logic
for caching feature that still remained.
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NOTE: My box has a DB containing 1284 files and 2973 timestamps.
`:Telescope frecency` takes around 20 milliseconds on average.
* I did an overhall for all codes and added typing by Lua-language-server and tests. It also works on CI.
* Now it searches files on the workspace completely asynchronously. It does not block your text input. (Fix#106)
Make count = 1 when you open a file you've never opened (Fix#107)
