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vinyl: write_iterator merges vlsns subsequnces

Now write iterator see only oldest vlsn. Because of it
the write iterator can not skip or merge or squash
newer statements, even if they are not used by any
transactions. For example, consider one key and its lsns:

      LSNs:  10   20   30   40   50
Read Views:  *         *         *

Statements with lsn 40 can be merged with 50;
20 can be merged with 30.
Old write iterator will merge nothing in that case,
because there is oldest vlsn = 10.

Lets allow the write iterator to see all read views, instead
of only oldest.
Such write iterator can merge multiple lsns subsequences
of each key.
For the example above it will merge in the following way:

      LSNs:   10   20   30   40   50
Read Views:   *         *         *
  Merge and  \__/\_________/\________/
    return:    merge,   merge,   merge
             and return each merged result.

For the considered case the difference is 3 result statements
instead of 5 by the old write iterator.

Closes #1920

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