Implement a rangeset data type and use it to track items in log segments.

Implement a rangeset data type and use it to track items in log segments.

The range set can be used to track which byte ranges in a file correspond
to valid objects, and so can be used in the cloud log code to check that
1) objects are actually available if partial log fetching is implemented,
and 2) that objects being fetched have actually been authenticated (if
needed).

This isn't fully working yet.  The range sets should be changed from a hash
table to a sequence, since some lookups in the cloud log code are into the
middle of an object (to skip the header and remap the data), and that needs
to be handled properly.