vinyl: check key uniqueness before modifying tx write set

Currently, we handle INSERT/REPLACE/UPDATE requests by iterating over
all space indexes starting from the primary and inserting the
corresponding statements to tx write set, checking key uniqueness if
necessary. This means that by the time we write a REPLACE to the write
set of a secondary index, it has already been written to the primary
index write set. This is OK, and vy_tx_prepare() relies on that to
implement the common memory level. However, this also means that when we
check uniqueness of a secondary index, the new REPLACE can be found via
the primary index. This is OK now, because all indexes are fully
independent, but it isn't going to fly after #2129 is implemented. The
problem is in order to check if a tuple is present in a secondary index,
we will have to look up the corresponding full tuple in the primary
index. To illustrate the problem, consider the following situation:

  Primary index covers field 1.
  Secondary index covers field 2.

  Committed statements:

    REPLACE{1, 10, lsn=1} - present in both indexes
    DELETE{1, lsn=2} - present only in the primary index

  Transaction:

    REPLACE{1, 10}

When we check uniqueness of the secondary index, we find committed
statement REPLACE{1, 10, lsn=1}, then look up the corresponding full
tuple in the primary index and find REPLACE{1, 10}. Since the two tuples
match, we mistakenly assume that there's a conflict.

To avoid a situation like that, let's check uniqueness before modifying
the write set of any index.

Needed for #2129