crimson: refactor asok command

crimson: refactor asok command

* do not define another iterator type, use `map::const_iterator`
  directly
* do not register hooks/commands with server block, register them
  one by one, much simpler this way.
* encapsulate the hook metadata in `AdminSocketHook`, so each
  `AdminSocketHook` instance is self-contained in the sense that
  we don't need to use an extra type for keeping track of them.

Signed-off-by: Kefu Chai <kchai@redhat.com>