ConfdInotifyEventHandler, move to a callback

ConfdInotifyEventHandler used to reload the config whenever a
notification arrived. Moving to a callback system, so that
ConfdConfigurationReloader can be responsible for that functionality.

Additionally the inotify class no longer reenables itself automatically,
but just notifies the callback if it's been disabled, and waits for its
call to enable(). This allows, should ConfdConfigurationReloader decide
it wants to move to polling, to avoid having a double enable()/disable()
call.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>