Change the internal representation in RangeSet.

This CL makes the following changes to RangeSet:
 - Uses std::pair<size_t, size_t> to represent a Range;
 - Uses std::vector<Range> to represent a RangeSet;
 - Provides const iterators (forward and reverse);
 - Provides const accessor;
 - 'blocks()' returns the number of blocks (formerly 'size');
 - 'size()' returns the number of Range's (formerly 'count').

Test: recovery_unit_test
Test: Apply an incremental update with the new updater.
Change-Id: Ia1fbb343370a152e1f7aa050cf914c2da09b1396