Change index; include atime; pack time as xstat timespec; use ns in memory.

Update the index format header to 'BUPI\0\0\0\4' (version 4).

Change the index to include atimes, and write all times to bupindex as
xstat timespecs.  This is in preparation for indexing all metadata.
After moving all of the times to the index, there should be far fewer
unique instances of the remaining metadata in many/most cases.

Change the index and index-related code to handle all time
values as integer nanoseconds since the epoch, excepting the packed
format, which (as mentioned above) is now an xstat timespec.

After these changes the index-related in-memory and on-disk time value
handling should match that of the metadata code (i.e. metadata.py,
.bupm, etc.).

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Reviewed-by: Zoran Zaric <zz@zoranzaric.de>
[rlb@defaultvalue.org: change 10e8 to 10**9 for ns per second.]