Move entity cache from web.request.ConnectionCubicWebRequestBase to repoapi.ClientConnection ResultSet._build_entity relies on an entity cache to avoid going round in circles (stack overflow due to infinite recursion) e.g. in a postcreate script. Clear the cache at commit/rollback time to avoid unexpected side effects; and explicitly clear the cache in a few tests where we mix ORM and RQL using the same connection.

[server] Fix AttributeError in extid2eid Session._tx was renamed in cb87e831c183, use _cnx instead.

[book] Update documentation for new repoapi Quite a few things change in 3.19: - repoapi instead of dbapi - ClientConnection / Connection / Session rework - web authentication process - test APIs Closes #3638793