Add token persistence optimization docs

[dushaniw]

Purpose

Related to wso2/api-manager#1664

Goals

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Approach

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User stories

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Release note

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Documentation

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Training

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Certification

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Marketing

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Automation tests

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Security checks

• Followed secure coding standards in http://wso2.com/technical-reports/wso2-secure-engineering-guidelines? yes/no
• Ran FindSecurityBugs plugin and verified report? yes/no
• Confirmed that this PR doesn't commit any keys, passwords, tokens, usernames, or other secrets? yes/no

Samples

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Related PRs

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Test environment

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Learning

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[CLAassistant]

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