Freeze and extract constants and config settings.

Freeze `Config#initialize` defaults. Change `Config#to_s` to use
symbols for `@listener`. Extract `Listener` job classes to
`Listener::FTP::SIDEKIQ_JOB_CLASS`,`Listener::IMAP::SIDEKIQ_JOB_CLASS`.
Rename `Notifier::Sidekiq::RESQUE_QUEUE` => `::SIDEKIQ_QUEUE`. Also
freeze various other small strings.