Embracing EventStore v2.0

- Added an IStreamUserCredentialsResolver to all configuration (with two
common implementations, i.e. fixed and no credentials).
- Integrated with the official v2.0 release (download that zip, extract
it properly and take on the necessary deps)
- Constructed a more realistic SingleVNode but with little success at
running it embedded
- Invested more in stubs & builders (mocking is on its way out of the
integration suite)
- Wild idea that builders might be useful beyond tests (need to ponder
that thought some more)
- Got rubbed in the face that RepositoryScenarioBuilder is long overdue
- Must make snapshot testing integrated instead of this mockery (doesn't
boost confidence)
- I can only start crying in advance when I'll have to merge all this
with my other branches / thoughts.