Integrate proxy APIs to IOTA service

[YingHan-Chen]

Currently, we have two proxy modes.
One is proxy_passthrough that "Pass proxy API directly to IRI without processing"

In another mode, TA will deserialize the request then serialize again, pass the request to IRI, deserialize the result then serialize again. I think deserialize-serialize then serialize-deserialize is meanless.

Also, without passthrough mode, we get errors when the serialize/deserialize the request/result to the Hornet. In passthrough mode, the proxy works fine.

[howjmay]

There are some core APIs not supported in the multiple serializing mode, and we do the serialization is actually for future optimization. We may apply caching or dcurl for the mode that we do serialization for several times.

However, setting proxy_passthrough as default mode may be a good idea.