Merge pull request #300 from yhmo/0.5.3

- \#204 - improve grpc performance in search

- \#207 - Add more unittest for config set/get

- \#208 - optimize unittest to support run single test more easily

- \#284 - Change C++ SDK to shread library

- \#284 - Change C++ SDK to shared library

- \#260 - C++ SDK README

## Task

### Build C++ SDK

The C++ SDK source code is under milvus/core/src/sdk. Build entire milvus project will also build the C++ SDK project. 

If you don't want to build the entire milvus project, follow below steps:

```shell

 # generate make files

 $ cd [Milvus root path]/core

 $ ./build.sh -l

 # build C++ SDK project

 $ cd [Milvus root path]/core/cmake_build

 $ make -C src/sdk

```

### Try C++ example

Firstly, you need to start a Milvus server.

If you've already built the entire milvus project, just start Milvus server with the following command:

```shell

 # start milvus server

 $ cd [Milvus root path]/core

 $ ./start_server.sh

```

You can also use Docker to start Milvus server:

```shell

 # pull milvus docker image and start milvus server

 $ docker pull milvusdb/milvus:latest

 $ docker run --runtime=nvidia -p 19530:19530 -d milvusdb/milvus:latest

```

Run C++ example:

```shell

 # run milvus C++ example

 $ cd [Milvus root path]/core/cmake_build/src/sdk/examples/simple

 $ ./sdk_simple

```

### Make your own C++ client project

Create a folder for the project, and copy C++ SDK header and library files into it.

```shell

 # create project folder

 $ mkdir MyMilvusClient

 $ cd MyMilvusClient

 # copy necessary files

 $ cp [Milvus root path]/core/cmake_build/src/sdk/libmilvus_sdk.so .

 $ cp [Milvus root path]/core/src/sdk/include/MilvusApi.h .

 $ cp [Milvus root path]/core/src/sdk/include/Status.h .

```

Create file main.cpp in the project folder, and copy the following code into it:

```shell

#include "./MilvusApi.h"

#include "./Status.h"

int main() {

  // connect to milvus server

  std::shared_ptr<milvus::Connection> conn = milvus::Connection::Create();

  milvus::ConnectParam param = {"127.0.0.1", "19530"};

  conn->Connect(param);

  // put your client code here

  milvus::Connection::Destroy(conn);

  return 0;

}

```

Create file CMakeList.txt in the project folder, and copy the following code into it:

```shell

 cmake_minimum_required(VERSION 3.14)

 project(test)

 set(CMAKE_CXX_STANDARD 14)

 add_executable(milvus_client main.cpp)

 target_link_libraries(milvus_client

         ${PROJECT_SOURCE_DIR}/libmilvus_sdk.so)

```

Now there are 5 files in your project:

```shell

MyMilvusClient

 |-CMakeList.txt

 |-main.cpp

 |-libmilvus_sdk.so

 |-MilvusApi.h

 |-Status.h

  ```

Build the project:

```shell

 $ mkdir cmake_build

 $ cd cmake_build

 $ cmake ..

 $ make

```

Run your client program:

```shell

 $ ./milvus_client

```

 No newline at end of file