[IMP] registry: switch for a dummy lock only during the execution of http tests
[IMP] tests.common: turn class methods into instance methods (this will ease some future refactoring)
[IMP] registry: add method get_cursor() to simply retrieve a new cursor, and refactor code to use it
[IMP] http tests: implement a test cursor that keeps a transaction open accross requests
 - TestCursor subclasses Cursor, and simulates commit and rollback with savepoints
 - the registry manages a test mode, in which it only uses the test cursor
 - a reentrant lock forces the serialization of parallel requests
[IMP] registry: now cursor() simply returns a cursor, which may itself be used as a context manager
[IMP] make TestCursor more robust, and remove some tests on tools.config['test_enable']
[IMP] sql_db: simplify TestCursor, given that method execute() does not need overriding
[IMP] registry: avoid every direct access registry.db, and rename attribute as registry._db
[FIX] registry: stupid typo
[MERGE] test_cursor: implement a cursor mockup that never commits to database, and use it for http tests