Mem compression occurred during the creation of the iterator

[liluo1990]

when leveldb create newRawIterator, it will do the following

first step:
em, fm := db.getMems()
second step:
v := db.s.version()

If first between second step, leveldb do a mem compaction。this will lead to the versions have the "fm or em data"。
I see the note NewMergedIterator ：
The input's key ranges may overlap, but there are assumed to be no duplicate
keys: if iters[i] contains a key k then iters[j] will not contain that key k.

Is this the expected design？