nixos/cockroachdb: simplify dataDir management, tweaks

This cleans up the CockroachDB expression, with a few suggestions from
@aszlig.

However, it brought up the note of using systemd's StateDirectory=
directive, which is a nice feature for managing long-term data files,
especially for UID/GID assigned services. However, it can only manage
directories under /var/lib (for global services), so it has to introduce
a special path to make use of it at all in the case someone wants a path
at a different root.

While the dataDir directive at the NixOS level is _occasionally_ useful,
I've gone ahead and removed it for now, as this expression is so new,
and it makes the expression cleaner, while other kinks can be worked out
and people can test drive it.

CockroachDB's dataDir directive, instead, has been replaced with
systemd's StateDirectory management to place the data under
/var/lib/cockroachdb for all uses.

There's an included RequiresMountsFor= clause like usual though, so if
people want dependencies for any kind of mounted device at boot
time/before database startup, it's easy to specify using their own
mount/filesystems clause.

This can also be reverted if necessary, but, we can see if anyone ever
actually wants that later on before doing it -- it's a backwards
compatible change, anyway.

Signed-off-by: Austin Seipp <aseipp@pobox.com>