HAMMER 60E/Many: Mirroring, bug fixes
* Work on the mirror_tid propagation code. The code now retries on
EDEADLK so propagation is guaranteed to reach the root.
* Get most of the mirror_write code working.
* Add PFS support for NFS exports. Change fid_reserved to fid_ext and use
it to store the localization parameter that selects the PFS. This isn't
well tested yet.
* BUGFIX: Fix a bug in vol0_last_tid updates. Flush sequences might
not always update the field, creating issues with mirroring and snapshots.
* BUGFIX: Properly update the volume header CRC.
* CLEANUP: Fix some obj_id's that were u_int64_t's. They should be int64_t's.
* CLEANUP: #if 0 out unused code, remove other bits of unused code.