memtx: fix MVCC breaking TREE index iterators

The MVCC transaction manager story garbage collection introduced a
subtle code dependency: TREE index iterators can get broken (see
definition of “broken” at struct bps_tree_iterator), because the
elements they are referencing can change during story garbage
collection.

We coined the notion “MVCC TRANSACTION MANAGER STORY GARBAGE
COLLECTION BOUND” to refer to this issue explicitly: iterators and the
tree elements they reference must not be used after this point in code.

Closes #6344

NO_DOC=bug fix