Add OpenSergoClientManager to support client reuse

[jnan806]

Signed-off-by: Jiangnan Jia jnan0806@gmail.com

Describe what this PR does / why we need it

support client reuse

Does this pull request fix one issue?

Fixes #8

Describe how you did it

• Add OpenSergoClientManager to manage the reuse OpenSergoClient
• Add Builder in OpenSergoClient for creating instance
• Add OpenSergoConfig to support customized config

Describe how to verify it

• for shared OpenSergoClient

  OpenSergoClient openSergoClient = OpenSergoClientManager.get().getOrCreateClient("127.0.0.1", 10246, new OpenSergoConfig());

• for prototype OpenSergoClient

  OpenSergoClient openSergoClient = new OpenSergoClient.Builder().endpoint("127.0.0.1", 
  10246).openSergoConfig(new OpenSergoConfig()).build();

Special notes for reviews

[sczyh30]

Thanks!