Cleaning up underused and non-recommended code

[jprobinson]

Some of the under-used features of Gizmo and the additional dependencies they bring in tends to be one of the things that keeps me up at night.

This PR will bring some breaking changes, but the hope is to cut down on our dependency footprint and hone the scope of the toolkit.

Work so far:

• web package is gone, important bits are pulled into server
• config/combined, config/cookie, config/gcp, config/oracle and config/mongodb are gone (let me know if you really need the latter two)
• config.LoadEnvConfig is gone as it was just a thin wrapper around envconfig.Process
• server.RPCServer is gone
• server.ESXHealthCheck is gone
• references to appengine in server are gone

[coveralls]

Coverage increased (+2.8%) to 45.822% when pulling 2019f79 on clean-up into c66e686 on master.