lib: always use 64-bit integers for json

lib: always use 64-bit integers for json

json-c does not (yet) offer support for unsigned integer types, and
furthermore, the docs state that all integers are stored internally as
64-bit. So there's never a case in which we would want to limit,
implicitly or otherwise, the range of an integer when adding it to a
json object.

Among other things this fixes the display of ASN values greater than
(1/2) * (2^32 - 1)

Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>