memtx: split and simplify memtx_tx_story_is_visible

Each story has two ends: the beginning and the end. For each
transaction both ends of a story could be visible or not.

Now there's a function that checks visibility of both ends of
a story. It can distinguish three cases: both ends are visible,
both ends are invisible, and the beginning is visible while the
end is not. The function returns true in the first and the last
cases; the actual case is clarified with an additional function
argument - visible_tuple, which is set to null in one of the
cases..

Let's make two different functions for checking visibility of
the beginning and the end of a story. Actually that is simple
split of function into two parts. The visible_tuple argument
will no longer be needed.

No logical changes.

Part of #8122

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring

(cherry picked from commit b6ef41ef)