SERVER-12461 Use larger values to test for too large $position arguments.

The previous values were integers that were just 3 away from the largest
possible 64-bit integer, were parsed as the same float as the largest possible
integer, and so were accepted.  This changes them to be much much larger.

Signed-off-by: Benety Goh <benety@mongodb.com>
SERVER-12461 Ensure LexNumCmp::cmp only returns 1, -1 or 0

strncmp's documentation says:

    It returns an integer less than, equal to, or greater than
    zero if s1 is found, respectively, to be less than, to match,
    or be greater than s2.

LexNumCmp::cmp is tested as returning only -1, 1 or 0 but returns the
return value of strncmp directly.  This changes it to clamp the return
value to -1, 1 or 0.

Signed-off-by: Benety Goh <benety@mongodb.com>