Integrate rpc and ptthttp

[chhsiao1981]

Version information:

Gptt
Version: 0.2.0
Git Commit: 33dac43
Architecture: amd64
Go Version: go1.11.1

Type:

Enhancement

Description:

Currently we have (json)rpc apis separated from other web-services due to the legacy issue～
We would like to integrate rpc into ptthttp as /api/rpc in ptthttp/server.go

[chhsiao1981]

The original purpose of rpc is to have a unified api-framework achieving the following goals:

1. Able to register arbitrary namespace / handlers.
2. Able to deal with arbitrary parameters.
3. Given an api-call, able to pass to the correct corresponding handler with correct corresponding parameters.

The drawback of the original rpc setup is that a new http-server is allocated internally in the lib.

We would like to have a method for ptthttp/server to handle the rpc call.