Currently the storage-service API handlers are set up in "random" place.
It can happen earlier -- as soon as the storage service itself is ready.
Also, despite storage service is stopped on shutdown, API handlers
continue reference it leading to potential use-after-frees or "local is
not initialized" assertions.
Fix both. Unsetting is pretty bulky, scylladb/seastar#1620 is to help.
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>