Move creation of knex instance to config module

addresses #2170
Only reference model properties through the models module.
This frees us up to enforce one single point of access, thus paving
the way towards allowing us to initialize the models at are request,
and not when it's require().

addresses #2170
Fix slowdown of tests by reverting back to using knex in test/utils
Merge pull request #3213 from hswolff/lazy-load-models

Preparation for lazy loading of models