admin_socket: initialize explicitly on startup; disallow changes

admin_socket: initialize explicitly on startup; disallow changes

There is an annoying dependency between the config lock and the admin
socket lock due to the fact that we initialize (or reinitialize) the socket
via a config observer.

Instead, explicitly initialize on startup.  Do not allow the admin socket
location to be changed at runtime.

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