Use strtou64_strict instead of atoi.
Fix bug with order_by(index="id")

Before if you ordered by the primary key then were actually treating it
like a sindex ordering due to extract_secondary being incorrect and
returning the primary key as if it were the secondary key. This is fixed
now.
Rewrites secondary key mangling.

The old method had lots of bugs in it. this new method is (hopefully) bug
free.

Also adds unittest for the secondary mangling functions.
Swap the order of SINGLE and MULTI.
Documents optarg in ql2.proto.
Don't check the results of as_datum() for nullity.