Attempt to make skydb more user-friendly

Automatically shut-down the sky_server when quitting.
Always start the skyserver at the mojo-root
instead of the parent directory to the path.
Log when starting a sky_server outside the mojo root.
Re-use already running sky_servers when possible.

R=esprehn@chromium.org, abarth@chromium.org

Review URL: https://codereview.chromium.org/679333005