space: add method to fetch next rowid

Ephemeral space are extensively used in SQL to store intermediate
results of query processing. To keep things simple, they feature only
one unique index (primary) which covers all fields. However, ephemeral
space can be used to store non-unique entries. In this case, one
additional field added to the end if stored data:

[field1, ... fieldn, rowid]

Note that it can't be added to the beginning of tuple since data in
ephemeral space may be kept as sorted. Previously, to generate proper
rowid index_max() was used. However, it is obviously wrong way to do it.
Hence, lets add simple integer counter to memtx space (ephemeral spaces
are valid only for memtx engine) and introduce method in vtab to fetch
next rowid value.

Needed for #3297