vinyl: disable deferred deletes if there are upserts on disk

Normally, there shouldn't be any upserts on disk if the space has
secondary indexes, because we can't generate an upsert without a lookup
in the primary index hence we convert upserts to replace+delete in this
case. The deferred delete optimization only makes sense if the space has
secondary indexes. So we ignore upserts while generating deferred
deletes, see vy_write_iterator_deferred_delete.

There's an exception to this rule: a secondary index could be created
after some upserts were used on the space. In this case, because of the
deferred delete optimization, we may never generate deletes for some
tuples for the secondary index, as demonstrated in #3638.

We could fix this issue by properly handle upserts in the write iterator
while generating deferred delete, but this wouldn't be easy, because in
case of a minor compaction there may be no replace/insert to apply the
upsert to so we'd have to keep intermediate upserts even if there is a
newer delete statement. Since this situation is rare (happens only once
in a space life time), it doesn't look like we should complicate the
write iterator to fix it.

Another way to fix it is to force major compaction of the primary index
after a secondary index is created. This looks doable, but it could slow
down creation of secondary indexes. Let's instead simply disable the
deferred delete optimization if the primary index has upsert statements.
This way the optimization will be enabled sooner or later, when the
primary index major compaction occurs. After all, it's just an
optimization and it can be disabled for other reasons (e.g. if the space
has on_replace triggers).

Closes #3638

NO_DOC=bug fix