An UPDATE operation is written as DELETE + REPLACE to secondary indexes.
We write those statements to the memory level even if the UPDATE doesn't
actually update columns indexed by a secondary key. We filter them out
in the write iterator when the memory level is dumped. That's what we
use vy_stmt_column_mask for.

Actually, there's no point to keep those statements until dump - we
could as well filter them out when the transaction is committed. This
would even save some memory. This wouldn't hurt read operations, because
point lookup doesn't work for secondary indexes by design and so we have
to read all sources, including disk, on every read from a secondary
index.

That said, let's move update optimization from the write iterator to
vy_tx_commit. This is a step towards removing vy_stmt_column_mask.