vinyl: fix assertion while recovering dumped statement

Certain kinds of DML requests don't update secondary indexes, e.g.
UPDATE that doesn't touch secondary index parts or DELETE for which
generation of secondary index statements is deferred. For such a request
vy_is_committed(env, space) may return false on recovery even if it has
actually been dumped: since such a statement is not dumped for secondary
indexes, secondary index's vy_lsm::dump_lsn may be less than such
statement's signature, which makes vy_is_committed() assume that the
statement hasn't been dumped. Further in the code we have checks that
ensure that if we execute a request on recovery, it must not be dumped
for the primary index (as the primary index is always dumped after
secondary indexes for the sake of recovery), which fires in this case.

To fix that, let's refactor the code basing on the following two facts:
 - Primary index is always updated by a DML request.
 - Primary index may only be dumped after secondary indexes.

Closes #4222