[transaction] keep a reference to the repo object

[session] make security_enabled API private The doc already says "internal API". The old entry point is now officially deprecated and the context manager have been made private.

[session] make hook_control API private The doc already says "internal API". The old entry point is now officially deprecated and the context manager have been made private.

[service] split session retrieval and service execution Internal session are not tracker by the repository and can not be retrieved using _getsession. Moreover their session id is not unique… Closes #2735700

[service] handle cnxset in the async case only The service may be called by a session, we really do not want the call to service to mess with the transaction state. This does not apply to async were thread magic ensure a dedicated transaction.

[service] extract session retrieval from "task" closure The session was retrieved there because `set_cnxset` must be called in the same thread as the code using it. We now retrieve the session beforehand and explicitly call `set_cnxset`.

[cubicweb/doc] Add tutorial on data import in CubicWeb. This involves creating the "tutorials/dataimport" directory structure under "cubicweb/doc" and, inside the "dataimport" directory, putting several files: - a ResT file containing the tutorial *per se*; this tutorial addresses the following issues: * creating a CubicWeb schema for representing a given data set (here, the Diseasome RDF data, for illustration purposes); * parsing the data; * importing the data, by using several stores: + the ``RQLObjectStore``, ``NoHookRQLObjectStore`` and ``SQLGenObjectStore`` from the ``dataimport`` module in CubicWeb; + the ``MassiveObjectStore`` from the ``dataimport`` module in the ``dataio`` cube. The tutorial also provides timing benchmarks of the various stores. - a set of Python files illustrating the data import, in the context of Diseasome RDF data parsing: * a Diseasome RDF data parse module, * a Diseasome data import module, * a CubicWeb schema for representing Diseasome data.

[dataimport] Slight message modification in exception handling code.

[dataimport] Handle various data formats when creating buffers from data. :note: On the long run, this should be addressed before, when these data are created.

[dataimport] Add a ``flush`` method for all stores. For ``ObjectStore``, ``RQLObjectStore`` and ``NoHookRQLObjectStore`` the method tries to call the ``commit`` method. For ``SQLGenObjectStore``, the ``flush`` method is basically unchanged with respect to previous changesets.