[server] add debugging for Hooks & Operations (closes #2470048)

[dataimport] Add SQL Store for faster import - works ONLY with Postgres for now, as it requires "copy from" command - closes #2410822 This store will use: - copy from for massive insertions. - execute from for update. The API of this store is similar to the other stores.

[test] swap order in assert of `test_users_and_groups_non_readable_by_guests` unittest2 order is (expected, got), not the opposite. This help test report readbility.

[test] fix server/querier/build_descr1 test. There is no self.transaction of TestCase. I assumed the test writer meant a session. This seems to make the test pass.

[server] make `test_bfss_fs_importing_transparency` less verbose on failure Dumping the whole content of a file on failure is a bit annoying. We only print the bad version here (it's usually the file path).

[entity] drop redundant cache update This ``cw_attr_cache`` update is redundant, ``_cw_update_attr_cache`` already do this step.

[cw-ctl] silence msgcat and msgfmt (closes #2527594)

Added tag cubicweb-debian-version-3.15.7-1 for changeset c5400558f370

Added tag cubicweb-version-3.15.7 for changeset d8916cee7b70

prepare 3.15.7