Add the dbus infinoted plugin

This plugin can at some point export an extensive interface to the
infinoted server via d-bus. At the moment, only three calls to query
and set ACLs are provided.

In the long term, this plugin can also replace the document-stream plugin,
by serving a file-descriptor over d-bus which would then be used to stream
the actual payload.
Make InfinotedLog thread-safe

InfinotedPluginLogging should now be made thread-safe, too.