memtx: fix logic of read tracker rebinding during rollback

During rollback read trackers are rebound to newer or older story, if
present. The problem with the current logic is that read trackers are
basically rebound in the first available index: all information about key
parts read from other indexes gets lost — rebind read trackers in all
indexes.

Closes #7828

NO_DOC=bugfix

(cherry picked from commit 559b27db)