hammer2 - redo the flush collision handling

hammer2 - redo the flush collision handling

* Add HAMMER2_CHAIN_ONRBTREE and change HAMMER2_CHAIN_DELETED.  The DELETED
  flag now indicates a chain has been deleted (as in unlink, rmdir, truncate,
  etc) but could not be removed due to a conflicting flush.

  DELETED means something different from dropped chains with 0 refs which
  wind up sticking around due to the lastdrop code not being able to
  acquire a lock on the parent or colliding with a flush.  0-ref chains
  can be considered to cache clean media state when it comes down to it.
  (modified chains have a ref and so don't hit the lastdrop code).

* The flush code is now able to reliably unlock the chain parent when
  processing a child.

* Clean up a number of flush cases.

* Began to add error handling in the hammer2_chain_create() path.  The path
  now handles EAGAIN errors when insertions would collide with a flush.
  (The wait/retry code is currently just a sleep/retry).

* Removed the MAYDELETE junk.  It was too junky.

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