move SLINK_DIRECTORIES creation to a function avoid unnecessary import (and kill that damed warning of psyco not being installed)

cleanup

[schema hooks] properly check for changes of schema properties avoid useless costly operation (such as full-text reindexing all entities of a type). Refactor inlined handling and fix a bug trying to reindex a final entity type on the way.

[hook] entity_oldnew_value may cause bug on attributes explicitly set to None

misc fixes to ensure logilab.db compatibility

[sql generation] fti_need_distinct quere was not anymore used, potentially causing duplicates on has_text queries with sqlite backend

[repo] enhanced security handling: deprecates unsafe_execute, in favor of explicit read/write security control using the `enabled_security` context manager. Also code executed on the repository side is now unsafe by default.

move hooks activation control on session object, so we can have a per transaction control. Added a new `hooks_control` context manager for usual modification of hooks activation.