vinyl: discard tautological DELETEs on compaction

The write iterator never discards DELETE statements referenced by a read
view unless it is major compaction. However, a DELETE is useless in case
it is preceded by another DELETE for the same key. Let's skip such
tautological DELETEs. It is not only a useful optimization on its own -
it will also help us annihilate INSERT+DELETE pairs on compaction.

Needed for #2875