box: snapshot should not include rolled back data

Box.snapshot() could include rolled back data in case synchronous
transaction ROLLBACK arrived during WAL rotation in preparation of
a checkpoint.

More specifically, snapshot consists of fixating the engines'
content (creation of a read-view), doing WAL rotation, and writing
the snapshot itself. All data changes after content fixation won't
go into the snap. So if ROLLBACK arrives during WAL rotation, the
fixated content will have rolled back data, not present in the
newest dataset.

The patch makes it fail if during WAL rotation anything was rolled
back. The bug sometimes appeared in an existing test about qsync
snapshots, but with a very poor reproducibility. In a new test
file it is reproduced 100% without the patch.

Closes #5167