phia: reduce pain in DDL

* Remove REF_FE/REF_BE magic code
* Ref count indexes in phia_cursor()
* Don't ref/unref count ALL indexes in phia_begin()/phia_tx_commit()
* Extract SI_SHUTDOWN/SI_DROP cases from si_execute()
* Fix handling of NO-OP task in planner
* Check sc_version after coeio call in phia_iterator_next()
* Add assertions and TRASH() to planner, index and tuples

Attempt to fix buggy phia_index refcouting. Now phia works without segfaults
more than 10 minutes on my host instead of 20-30 sec. DDL still may crash
in some situations.