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vinyl: fix handling of overwritten statements in transaction write set

Statements executed in a transaction are first inserted into the
transaction write set and only when the transaction is committed, they
are applied to the LSM trees that store indexed keys in memory. If the
same key is updated more than once in the same transaction, the old
version is marked as overwritten in the write set and not applied on
commit.

Initially, write sets of different indexes of the same space were
independent: when a transaction was applied, we didn't have a special
check to skip a secondary index statement if the corresponding primary
index statement was overwritten because in this case the secondary
index statement would have to be overwritten as well. This changed when
deferred DELETEs were introduced in commit a6edd4559bca4 ("vinyl:
eliminate disk read on REPLACE/DELETE"). Because of deferred DELETEs,
a REPLACE or DELETE overwriting a REPLACE in the primary index write
set wouldn't generate DELETEs that would overwrite the previous key
version in write sets of the secondary indexes. If we applied such
a statement to the secondary indexes, it'd stay there forever because,
since there's no corresponding REPLACE in the primary index, a DELETE
wouldn't be generated on primary index compaction. So we added a special
instruction to skip a secondary index statement if the corresponding
primary index was overwritten, see `vy_tx_prepare()`. Actually, this
wasn't completely correct because we skipped not only secondary index
REPLACE but also DELETE. Consider the following example:

```lua
local s = box.schema.space.create('test', {engine = 'vinyl'})
s:create_index('primary')
s:create_index('secondary', {parts = {2, 'unsigned'}})

s:replace{1, 1}

box.begin()
s:update(1, {{'=', 2, 2}})
s:update(1, {{'=', 2, 3}})
box.commit()
```

UPDATEs don't defer DELETEs because, since they have to query the old
value, they can generate DELETEs immediately so here's what we'd have
in... (continued)

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