[registry] refactoring into a abstract and factory

[registry] refactoring into a abstract and factory

* a new abstract class AbstractAccessor is the new base
  for all concrete database implementations.
* a factory now, dynamically discovers those implementations
  in the registry/ directory and will create the configured
  type for the plugins.
* again: this makes db keys case-sensitive (aka 'the correct
  way of doing things' -.-)
* re-added tokyocabinet