Refactor Listeners, pushing #check_all logic into App .

Move `Listener::FTP#check` => `#get_unseen_files`, moving `#check_all`
=> `#check` changing to check single dir. Update `App#check_all_ftp` to
loop the dirs.

Move `Listener::IMAP#check` => `#get_unseen_uids`, moving `#check_all`
=> `#check` changing to check single mailbox. Update `App#check_all_imap`
to loop the mailboxes.

Extract Redis key construction to `Listener::FTP#redis_key`,
`Listener::IMAP#redis_key`
Refactor Notifier::Sidekiq , extracting to #sidekiq_job and logging.

Extract Sidekiq job payload to `Notifier::Sidekiq#sidekiq_job`. Add
`SIDEKIQ_KEY_QUEUES`,`SIDEKIQ_KEY_QUEUE_X` extractions of Redis key
constants. Add logging.

Note that `App#notifiers` already attached `:logger`, although it
previously had no effect because `Notifier::Sidekiq` did not log.