vinyl: prepare write iterator heap comparator for deferred DELETEs

In the scope of #2129, we won't delete the overwritten tuple from
secondary indexes immediately on REPLACE. Instead we will defer
generation of the DELETE statement until the primary index compaction.
However, it may happen that the overwritten tuple and the tuple that
overwrote it have the same secondary key parts, in which case the
deferred DELETE is not needed and should be discarded on secondary
index compaction. This patch makes the write iterator heap comparator
function discard such useless deferred DELETEs.

Note, this patch also removes the code that prioritises terminal
statements over UPSERTs in the write iterator, which, according to the
comment, may happen only during forced recovery. I don't see why we
should do that, even during forced recovery, neither have I managed to
find the reason in the commit history, so I dropped this code in order
not to overburden the write iterator logic with some esoteric cases.

Needed for #2129