mon: return instead of exit(3) via preforker

This lets us run all the locally-scoped dtors so that leak checking will
work.

Signed-off-by: Sage Weil <sage@inktank.com>
mon: fix leak of health_monitor and config_key_service

Switch to using regular pointers here.  The lifecycle of these services is
very simple such that refcounting is overkill.

Signed-off-by: Sage Weil <sage@inktank.com>