Fixing possible losing data changes:

1. New flag - BM_JUST_DIRTIED - added for BufferDesc;
2. All data "dirtiers" (WriteBuffer and WriteNoReleaseBuffer)
   set this flag (and BM_DIRTY too);
3. All data "flushers" (FlushBuffer, BufferSync and BufferReplace)
   turn this flag off just before calling smgr[blind]write/smgrflush
   and check this flag after flushing buffer: if it turned ON then
   BM_DIRTY will stay ON.