Remove old local-locking strategy

This should make the diff for adding the new one smaller.
Use a distributed lock for fact check processing

This means that we've got (some) guarantee that only one process is accessing the email
at a given time.
FactCheckEmailHandler#process takes a block
Renew the lock after processing each message

This should ensure that we retain the lock, even if processing the set takes
longer than five minutes.
Add missing newline
Use a released version of redis-lock

The Pull Request was merged, yay!
Merge pull request #91 from alphagov/prevent_concurrent_fact_check_processing

Prevent concurrent fact check processing