[repo] straightforward bootstrap sequence. Closes #2841188
* kill fill_schema method which was doing to much different things, in favor of
a deserialize_method which is simply reading schema from the db and returning
it
* set_schema doesn't bother with cubes
This allows to easilly follow what's going on and so to cleanup init_cnxset_pool
to get the following boostrap order:
1. load cubes (implies loading site_cubicweb)
2. load schema
3. init data sources
4. init cnx sets
[repo] minor cleanups to bootstrap sequence
* fix some docstring/comments
* move initialization of HooksManager to the __init__.py, this has nothing to
do with pool initialization / boostraping