Resolve "Run as many gdk doctor checkups in parallel as possible"

What does this Merge Request do?

This MR does the following:

• Updates GDK doctor checks to be run in parallel to speed up execution
• Changed from gdk doctor running gdk start to just gdk start postgresql again to speed up execution
• Updated how the results of gdk status are interpreted so we only flag a 'down' service if it's meant to be 'up'

Previously

$ time gdk doctor
gdk doctor  19.15s user 7.79s system 67% cpu 40.052 total

Now

$ time gdk doctor
gdk doctor  19.20s user 7.52s system 161% cpu 16.593 total

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• [-] Documentation added/updated, if needed.
• [-] gdk doctor test added, if needed.

Closes #777 (closed)