加速MDC表的加载速度

- the insert code to trust the FULL bits in the block indexes

  even though this is just a hint. So only the first pass of

  the block index will be done. The first pass will only

  visit blocks which do not have the FULL bit set.

  If the visited blocks are found to be full, the FULL bit

  will be set in the block index. The second pass of the

  block index, which visits blocks even if the FULL bit is

  set, and which clears the FULL bit when space is found,

  will not be done.

- This will speed up the insert.

- The side affect of this registry variable is that until an

  entire block is deleted, any space freed by deletes within

  those blocks will not be reused.