Previously we accessed offsets in shared MultiXactState without locks as 32-bit read is always atomic. But I'm not sure it's so when offset become 64-bit.
E.g. GetNewMultiXactId():
nextOffset = MultiXactState->nextOffset;
is outside lock.
There might be other places we do the same as well.
I think the replacement of plain assignments by pg_atomic_read_u64/pg_atomic_write_u64 would be sufficient.
