fix!: change timing for initialization

[delphinus]

fix https://github.com/nvim-telescope/telescope-frecency.nvim/issues/109 fix https://github.com/nvim-telescope/telescope-frecency.nvim/issues/59

This fixes problems below.

• Auto-validation feature is called at Neovim starting.
  • → Now it starts at :Telescope frecency called at the first time.
• frecency.setup() is called every when :Telescope frecency is called.
  • → Now it is called only once.
• telescope.setup() calls frecency.new() and it reads the database. This causes time in executing init.lua.
  • → Now it reads the database lazily. It reads at the first time when needed.