memtx: track `index:random` reads and clarify result

TREE (HASH) index implements `random` method: if the space is empty from
the transaction's perspective, which means we have to return nothing, add
gap tracking of whole range (full scan
tracking), since this result is equivalent to `index:select{}`, otherwise
repeatedly call `random` and clarify result, until we get a non-empty one.
We do not care about performance here, since all operations in context of
transaction management currently have O(number of dirty tuples)
complexity.

Closes #7670

NO_DOC=bugfix