txm: rewrite and refactor mvcc code

There were several problems that was connected with broken
pointers in tuple history. Another problems is that that code
was quite huge and difficult to understand.

This patch refactors all the code that is connected to lists of
stories in history. A bunch of helper function was added and in
fact these functions was carefully rewtitten:
 * memtx_tx_history_add_stmt
 * memtx_tx_history_rollback_stmt
 * memtx_tx_history_prepare_stmt
 * memtx_tx_history_commit_stmt

In addition to refactoring a couple of significant changes was
made to the logic:
 * Now del_story in statement point to story of the tuple that
was effectively deleted by this statement.
 * Conflicts in secondary indexes (that were previously named as
'cross coflicts' now handled transparently during statement
preparation.

Closes #6132
Closes #6021