1. LocalBufferSync(): set nextFreeLocalBuf to 0 - we're freeing

   all local buffers @ xact commit, so accordingly nextFreeLocalBuf
   is first local buffer now.
   It helps to avoid unnecessary local buffer allocations in LocalBufferAlloc()
   latter ("memmory leaks" in 'order by').
2. ResetLocalBufferPool() lost allocated local buffers:

   memset(LocalBufferDescriptors, 0, sizeof(BufferDesc) * NLocBuffer);

   (local buffers leak @ xact aborts).