kernel - Remove most buffer_map contention on 64-bit systems

* Set BKVASIZE to MAXBSIZE (65536) on 64-bit systems.  This has the effect
  of always reserving a maximal amount of KVM for each buffer cache buffer.

* The change removes most buffer_map interactions once the system caches
  have stabilized.

* The change removes the need to defragment the buffer cache.

* Significant performance improvement for HAMMER1 and HAMMER2 which use
  larger buffers and were hitting degenerate fragmentation issues before
  this change.

* But also results in lower buffer data density when buffering data for
  smaller files, so may have a slight detrimental effect on UFS and on
  the amount of time dirty data can be cached before being flushed to
  disk.

* NOTE: The 64K limit is for normal buffers and is unrelated to the
 	physical cluster buffer (pbuf) limit of 128KB (MAXPHYS).
kernel - rename vm_map lock wmesg

* Rename the vm_map lock wmesg from "thrd_sleep" to "vm_maplk" to reduce
  confusion when observing 'ps' output.
hammer2 - Stabilization

* Fix bugs where cached values in the chain's shared core were being
  used and modified during a flush by dead chains.  These values
  are only supposed to be used by live chains.  Should fix a number of
  assertion panics.

* Fix bug where the live_count calculation in a chain's shared core could
  be made based on a dead chain's block table.  This calculation is also
  meant to only be made/used by live chains.