refactor: use setClient and getClient of core

[mojito317]

PR summary

The purpose of this PR was originally calling and returning the result of core's invokeRequest from our requestToken method, but the convert method called in invokeRequest takes variables only from the response body. With this feature response headers which are essential to CouchDBSessionAuthenticator cannot be reverse engineered from invokeRequest.

Although I could change sessionAuthClient with getter and setter methods introduced in IBM/java-sdk-core#174.

PR Checklist

Please make sure that your PR fulfills the following requirements:

• The commit message follows the
  Angular Commit Message Guidelines.
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• New tests
• Build/CI related changes
• Documentation content changes
• Other (please describe)

What is the current behavior?

sessionAuthClient is set and being used from the requestToken method.

What is the new behavior?

The client of the class is set and being used from the requestToken method.

Does this PR introduce a breaking change?

• Yes
• No

Other information

[ricellis]

+1

[mojito317]

Thanks for the review @ricellis! I am about to merge this PR.