Sane ApiClient (concurrency/resource usage improvements)

[sabirove]

This PR aims to solve/provide a workaround for

1. ApiClient concurrency issues (e.g: Thread Safety - either fix or document #123)
2. Inefficient resource utilization issues (e.g. new http Client and ObjectMapper allocated per each new client with no way to provide those from the outside, thus avoiding concurrency issues by creating new instance is not an option. On top of that new http Client is allocated per each method call (!!!) for certain methods, see ApiClient::buildHttpClient).

Note:

• Changes are not braking and backwards compatible
• ApiClient API is intact. New API (ApiClientFactory) is added to address the issues
• Java version bumped to 1.8 (README.MD specifies requirement as 1.9+ anyway)

[sabirove]

@mmallis87 please take a look.

[sabirove]

Build fails with

SdkUnitTests.setUp:68->getLastTenEnvelopeIds:1825 Exception: rsaPrivateKey byte array is empty

because this statement in SkdUnitTests

private static final byte[] privateKeyBytes = Base64.decode(System.getenv("PRIVATE_KEY"));

yields empty byte array.
Not sure this has something to do with PR changes, looks more like a CI/env config related issue.

https://app.travis-ci.com/github/docusign/docusign-esign-java-client/jobs/529199291