vinyl: add frozen mems of splitting range to read iterator

Currently, if the range is splitting, we only add active in-memory
indexes of the resulting ranges to the read iterator, see
vy_read_iterator_add_mem(). This is because until recently a mem could
only be frozen on dump/compaction task preparation, which is disabled
while split is in progress. However, it is not true any more - a mem can
be rotated on txn_commmit() in case of DDL, hence we must always add all
in-memory indexes, including frozen ones, when opening a read iterator.