memtx: replace conflict trackers with read trackers

Conflict trackers are used to store information that if some
transaction is committed then some another transaction must be
aborted. This happens when the first transaction writes some
key while the other reads the same key. On the other hand there
are another trackers - read trackers - that are designed to
handle exactly the same situation. That's why conflict trackers
can be simply replaced with read trackers.

That would allow to remove conflict trackers as not needed
anymore.

Part of #8648
Part of #8654

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring