Soprano 2 TODO List

Add here what you want to add in Soprano

* threaded async inference: run inference in a different thread to make add and remove calls return quickly (as an option which can be configured)
* some reference counting in the inference model to properly handle removal of statements

Soprano 3:
* make Soprano::Backend::createModel non-virtual and let it call a virtual protected method. Then remove the Backend* paramter
  from the StorageModel constructor and have Backend set the Backend and the BackendSettings in Backend::createModel
* Make Soprano::Server::ServerCore::createModel const.
