[Feature Request] Add support for mysql backend to persist trace data

[ayansen]

As of now haystack-traces just supports a memory and a cassandra backend for storage. We moved the storage code to a separate sidecar app as part of another issue ExpediaDotCom/haystack-traces#185.
It would be nice to have a mysql backend for teams running mysql clusters or using AWS Managed RDS

[shreyaa-sharma]

PR still open here, I think only the readme is to be updated. Tagging @ayansen and @abhinavtripathi7.