memtx: rename nearby_gaps -> read_gaps

In further commit this list will be used for tracking all read
gaps, not only 'nearby'. Since this rename has rather huge
diff, let's make it in separate commit.

No logical changes.

Part of #8648
Part of #8654

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring

(cherry picked from commit d3feb691)