memtx: simplify check_dup function

There ware two functions - check_dup_clean and check_dup_dirty.
Merge them to one.

Also extract phantom checks from check_dup and call them explicitly.
That will additionally simplify check_dup and will allow to get rid
of temporary conflict trackers - memtx_tx_conflict. Note that this
kind of object will remain in memory monitoring by now. It will be
removed later.

No logical changes.

Part of #8122

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring

(cherry picked from commit a5dd48ed)