persisting measured jobs in redis

[shivakumaarmgs]

Linking #32

Reason: Without this change, only one or two job stats are shown on the page and they too are gone if we restart the workers. This is because the measured jobs class names are stored in an instance variable.

Made changes to store the measured jobs names in redis so that Job_Stats page shows the stats for all the Jobs ever recorded. This will fix #26.

Also fixing #21.
Need idea on solving the following scenarip:
A super-class extends just the JobStats::Duration module and if that super-class is inherited by a sub-class, that sub-class gets extended by JobStats module instead of JobStats::Duration module.

These changes are currently being used in our production without any issues.

[baarkerlounger]

@lukeasrodgers are you maintaining this repo? Did you ever get a chance to look at this?

[lukeasrodgers]

@baarkerlounger I am still maintaining it but (obviously) not super actively and haven't been able to put much time into reviewing this PR, was initially a bit leery about running the const_get on whatever is in redis, which is a possible security vulnerability, e.g. see https://blog.convisoappsec.com/en/exploiting-unsafe-reflection-in-rubyrails-applications/ and https://www.praetorian.com/blog/ruby-unsafe-reflection-vulnerabilities?edition=2019

I appreciate the problem this PR is solving but I think the security risks are not worth it here.

Thanks @shivakumaarmgs and very sorry for the delay here.