Make the DB client a static var

[FoamScience]

Principle: Static members are initialized at global space; before starting the main function. So, each process can have a shared client for all smartSim function objects it instantiates.

Drawback: No user customization, we're stuck with a client where cluster mode is turned off.

More artifacts:

• There is a Dockerfile for a dev docker image;
• an attempt to use that image to unit-test this PR.
• SmartRedis subtree has been updated to latest develop commit