- add forgotten check of lsn for insert completion
- remove level of pages: hard to check in recovery
- some cleanups

- full concurrency for insert/update/select/vacuum:
        - select and vacuum never locks more than one page simultaneously
        - select (gettuple) hasn't any lock across it's calls
        - insert never locks more than two page simultaneously:
                - during search of leaf to insert it locks only one page
                  simultaneously
                - while walk upward to the root it locked only parent (may be
                  non-direct parent) and child. One of them X-lock, another may
                  be S- or X-lock
- 'vacuum full' locks index
- improve gistgetmulti
- simplify XLOG records

Fix bug in index_beginscan_internal: LockRelation may clean
  rd_aminfo structure, so move GET_REL_PROCEDURE after LockRelation

2. improve vacuum for gist
   - use FSM
   - full vacuum:
      - reforms parent tuple if it's needed
        ( tuples was deleted on child page or parent tuple remains invalid
          after crash recovery )
      - truncate index file if possible
3. fixes bugs and mistakes

recovery after crash (power loss etc) it may say that it can't restore
index and index should be reindexed.

Some refactoring code.