Config reloading

[Sergei Lebedev]

Each application module implicitly subscribes to a particular config section in nakaz:use call (see #9), once this section changes, the module is notified of this via a behaviour callback, details:

• a module must define nakaz behaviour callback, if it calls nakaz:use at least once;
• two callbacks, one for checking that the module can reload the config, and one for the actual reloading;
• two strategies -- sync (only reload if all section watchers are ready) and async (reload right away).