Add purge log service

The purge log service is responsible for removing the content of logs
after they have been archived to S3.
Add database method required for purge_log service
purge_log: Warn if log is not in database or on S3
purge_log: Re-archive log if S3 doesn't match database
Don't log SQL statements
Bundle log purging into a single database call
purge_log: Run multiple DB calls in a transaction
Content length should be nil if status wasn't success
1.7.6 plz
some metrics, some logging, and a smaller run method
moar 1.7.6
measure not mark, duh
moar @ needed
so many moar @ needed
fetch the log content length from the db, instead of the content, then calculate the length

some refactorings to the purge service to work with these changes
correct the purge log specs
add a purge log sidekiq job, and wire it up in the procfile
does using a symbol make a difference?
using a prepared statement the sequel way just didn't play nice with the use of the char_length sql function
Update .ruby-version to match Gemfile