interval_map: Add interfaces for getting start/end and lower ranges.

interval_map: Add interfaces for getting start/end and lower ranges.

 These new utilities add the ability to:
   get_lower_range: Return the lowest interval iterator which covers the specified range
   get_start_off(): return the first offset in the interval.
   get_end_off(): Return the end offset of the last interval
   contains(): Return true if specified is entirely contained within the interval map.

Signed-off-by: Alex Ainscow <aainscow@uk.ibm.com>