vinyl: fix assert in vy_tx_write()

Assert in vy_tx_write() validates that upsert applied on the top of
cached statement is always replace. In fact, it is not always so. If
vy_apply_upsert() fails due to the fact that PK is modified, it returns
old statement (i.e. statement which upsert is applied on). In this
regard, if tuple cache contains insert and invalid upsert is executed,
vy_apply_upsert() will return insert. As a result, assert will fire.
Let's fix it and take into account that vy_apply_upsert() is able to
return inserts as well.

Closes #5005