hammer2 - Add indirect block creation

hammer2 - Add indirect block creation

* hammer2_chain_create_indirect() is now called by hammer2_chain_create()
  when the current parent is full and an indirect block is needed to
  open up new slots.

  This function scans the key/keybits for all the entries in the parent and
  calculates the key/keybits representing the range, inclusive of the
  key/keybits we were trying to insert when we ran out of room.

  The range is then bisected and the low or high side is selected for
  indirect block replacement.  The indirect block is created and the low
  or high side elements are moved into the indirect block.  The function
  then returns and hammer2_chain_create() retries.

* Implement the HAMMER2_CHAIN_MOVED flag.  This flag is set in a chain
  element that has been moved into an indirect block, allowing flushes
  to detect the condition and update the element in the indirect block
  during the flush operation.

  We also subtley use NOLOCK in a few places since there is no need to
  actually load the contents of the entries being moved.

* Correct issues related to iterating through indirect blocks that came
  up in testing.

* Tested with mkdir > 8 entries.