vinyl: don't eliminate insert+delete during index build

In 0e37af31 an optimization eliminating INSERT+DELETE and DELETE+INSERT
statements by the same key in write set was introduced. It is fine until
it comes for secondary index build. While we are building secondary
index we save current lsn, set on_replace trigger forwarding new
requests to the secondary index and copy row-by-row tuples (to be more
precise keys) to secondary index until lsn of tuple is less than the one
we preserved at the start. Now, if during index build we execute request
replacing key that hasn't been already transferred to secondary index,
we will get missing key in secondary index since:
a) In on_replace trigger replace is split into DELETE+INSERT and
   eliminated by mentioned optimization (the same concerns simple pair
   of DELETE+INSERT requests made in single transaction - so that they
   get into one write set);
b) It is skipped in the main loop transferring tuples from PK to SK
   since lsn of modified tuples is greater than saved lsn.

In this respect, we may get missing tuples in secondary index.
The proposed solution is quite trivial: we are able to track that index
is still being created (see previous commit) so we won't apply
INSERT+DELETE annihilation if index build is not finished.

Closes #6045