Expand the range of integer precision when converting into/out of json.

For MESOS-3299, we added some protobufs to represent time with integer precision. However, this precision is not maintained through protobuf <-> JSON conversion, because of how our JSON encoders/decoders convert numbers to floating point.

To maintain precision, we can try one of the following:

• Try using a long double to represent a number.
• Add logic to stringify/parse numbers without loss when possible.
• Try representing int64_t as a string and parse it as such?
• Update PicoJson and add a compiler flag, i.e. -DPICOJSON_USE_INT64

In all cases, we'll need to make sure that:

• Integers are properly stringified without loss.
• The JSON decoder parses the integer without loss.
• We have some unit tests for big (close to INT32_MAX/INT64_MAX) and small integers.