memtx: force async snapshot transactions

Snapshot rows contain not real LSNs. Instead their LSNs are
signatures, ordinal numbers. Rows in the snap have LSNs from 1 to
the number of rows. This is because LSNs are not stored with every
tuple in the storages, and there is no way to store real LSNs in
the snapshot.

These artificial LSNs broke the synchronous replication limbo.
After snap recovery is done, limbo vclock was broken - it
contained numbers not related to reality, and affected by rows
from local spaces.

Also the recovery could stuck because ACKs in the limbo stopped
working after a first row - the vclock was set to the final
signature right away.

This patch makes all snapshot recovered rows async. Because they
are confirmed by definition. So now the limbo is not involved into
the snapshot recovery.

Closes #5298