Harmless renaming.

New class RootDB has no parents and handles RPC calls to the cc-server in a
transparent fashion from its children.

Support for registering/unregistering sub object to the cc-server.
Support for tag lifecycle handling (libev), was previously managed by plugins.

Plus error handling.

Refactored plugins.
Updated hypervisor libvirt event handling.
Async rpc handling.
Minor renaming for libvirt event callback.
Remove uneeded method sub_tags (in hypervisor Handler).