The interval template member functions mostly accept
tri-comparators but a few functions accept less-comparators.
To reduce the chance of error, and to provide better error
messages, constrain comparator parameters to the expected
signature.
In one case (db/size_estimates_virtual_reader.cc) the caller
had to be adjusted. The comparator supported comparisons
of the interval value type against other types, but not
against itself. To simplify things, we add that signature too,
even though it will never be called.
Closes#9291
16116ac631