HAMMER 55: Performance tuning and bug fixes - MEDIA STRUCTURES CHANGED!

HAMMER 55: Performance tuning and bug fixes - MEDIA STRUCTURES CHANGED!

* BUG-FIX: Fix a race in hammer_rel_mem_record() which could result in a
  machine lockup.  The code could block at an inappropriate time with both
  the record and a dependancy inode pointer left unprotected.

* BUG-FIX: The direct-write code could assert on (*error != 0) due to an
  incorrect conditional in the in-memory record scanning code.

* Inode data and directory entry data has been given its own zone as a
  stop-gap until the low level allocator can be rewritten.

* Increase the directory object-id cache from 128 entries to 1024 entries.

* General cleanup.

* Introduce a separate reblocking domain for directories: 'hammer reblock-dirs'.