Add initial draft for IStorage-level watch API and implemention:

- Notifier goes away;
- Add Watcher interface instead with AddWatch/DelWatch calls. See
  Watcher documentation for details.
- IStorage wrapper now subscribes to driver events, retranslate them
  to users and handles Add/Del watch subscription requests.

XXX Raw cache becomes temporarily disabled, until it is fixed to handle invalidations.
XXX Tests pending.