cli/demo: refactor the server initialization

The latency simulation code needs to be injected as a latency map
while the servers are initialized, i.e. concurrently with server
startup.

The previous initialization code for `cockroach demo` to achieve this
was exceedly difficult to understand and was, in fact, incorrect.

This commit reworks this code by exposing the overall workings
as a comment and then ensuring the structure of the comment follows
the explanation. It also add logging.

Additionally, this change ensures that the same initialization code is
used regardless of whether latency simulation is requested or not.

Release note: None

Co-authored-by: Raphael 'kena' Poss <[email protected]>
Co-authored-by: Oliver Tan <[email protected]>