Added persistent-server contribution to always keep server open

Stops server from closing when quiting.
Function defined by spacemacs should use `/` instead of `-` (reserved for variables).
So the function name should be: `spacemacs/save-buffers-kill-emacs`

Same thing applies for the other function names in the PR.

More info here: https://github.com/syl20bnr/spacemacs/blob/master/doc/CONVENTIONS.md#spacemacs-core-and-layer