Fixed issues for Refresh Token auth flow.

[ashishdhingra]

Issue #: #76 #77 #75

Description of changes: Following issues are addressed in this PR:

• Removed check to validate CognitoSessionTokens which checks ExpirationTime for REFRESH_TOKEN Auth Flow.
• Fixed an issue where IssuedTime and ExpirationTime for CognitoUserSession object should be in UTC when it is instantiated manually by user.

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.