HAMMER VFS - Change default from version 3 to version 4
* Version 4 is now the default for newfs_hammer and is no longer
considered a work in progress.
* Filesystems may be upgraded to version 4 in-place. The UNDO FIFO
(typically 1G) is reformatted so the upgrade might take a minute or
two depending.
* Version 4 allows the UNDO FIFO to be flushed without also having
to flush the volume header, removing 2 of the 4 disk syncs typically
required for an fsync() and removing 1 of the 2 disk syncs typically
required for a flush sequence.
This is accomplished by adding a sequence number of the UNDO entries
in the FIFO and preventing them from crossing a 512-byte boundary,
so each 512-byte block starts with an entry. HAMMER's mount code
is then able to scan the UNDO FIFO to determine the start and end
for recovery purposes.
* Version 4 implements the infrastructure needed for REDO, but does not
implement any REDO operations.