eliminate redis & mongodb

[derekperkins]

I know this is very presumptuous of a newcomer, but I would strongly recommend eliminating the redis + mongodb dependencies from the stack. MySQL is more than capable to handle almost any workload you would use the other two for, and less infrastructure is always easier to manage.

FWIW, I'm a core maintainer of the https://github.com/vitessio/vitess project and have operated sharded MySQL at high scale, so I am confident in MySQL's ability to handle those workloads.