[REF] registry: always perform registry/cache signaling at the end of request

Problem: the update of custom models/fields is not fully transactional, and may
potentially lead to an inconsistent database.  An other problem is creating two
custom fields by writing on a model: if the second one fails, the first one has
been committed without notice.  Retrying the request will give an unexpected
error (duplicate field name).

Solution: never commit in the middle of a request.  If the changes have an
impact on the registry, then mark it as invalid (with a new flag), and signal
registry invalidation after everything has been committed.  If the request
fails, reset the registry.  Both registry and cache invalidation are handled
the same way.

odoo/tools/sql.py

@@ -189,7 +189,6 @@ def drop_index(cr, indexname, tablename):
 
 def drop_view_if_exists(cr, viewname):
     cr.execute("DROP view IF EXISTS %s CASCADE" % (viewname,))
-    cr.commit()
 
 def escape_psql(to_escape):
     return to_escape.replace('\\', r'\\').replace('%', '\%').replace('_', '\_')