Add new lock

• Remove unique_lock and use unique: to set this like in unique: :until_timeout
• Warn when using unique: true and suggest to change it to what we need with a fallback.
• Create constants for hash keys to avoid having to fix spelling or for renaming keys only having to be done in one place and avoid having to type .freeze everywhere.
• Move all explicit logic out from the server middle ware and make it just call execute on the instance of the lock class (prepare for allowing custom locking classes to be used).
• Create a new job for scheduling jobs after it started executing but only allow one job to run at the same time.