If the first argument of box.atomic is a non-callable table then
consider it as options table for box.begin{}.

For test and debug purposes introduce internal getter of current
transaction isolation level as box.internal.txn_isolation().

Closes #7202

@TarantoolBot document
Title: Options in box.atomic

Now it's allowed to pass transaction options in the first argument
of box.atomic(..) call. The options must be a table, exactly as
in box.begin(..). If options are passed as the first arguments,
the second and the rest arguments are expected to be a functions
and its arguments, like in usual box.atomic.