HAMMER 4/many - more core infrastructure

HAMMER 4/many - more core infrastructure

* Add reserved areas for a boot area and a memory log.

* Add merged scan operations which are the core procedures used to execute
  most filesystem operations.  These functions will access both the
  in-memory tree of unsynchronized information and the on-disk topology
  to generate a 'merged' result.

  Amoung other things this allows the filesystem to hold operations in a
  memory cache without actually having to mess with the HAMMER topology
  on-disk.  The on-disk topology is then updated in a deferred manner.
  Disk I/O is entirely avoided for self contained operations which create,
  write, and delete related files quickly enough.

* Add unmount sequencing, make mount and df work again.

* Test the reference counting and flushing system on most primary
  structures.

* Test basic buffer cache interactions, reading, writing, and lazy
  synchronization.

* Start tying VNOPS into the infrastructure.  open/close/read/write works
  now via the in-memory cache (none of it is synched to the disk topology
  yet!).  readdir doesn't yet work... the 32 bit cookies are not large
  enough.