Add abstraction dao layer for token revoke flow

[chamilaadhi]

Proposed changes in this pull request

• As a part of removing token persistence efforts in Removal of Token Persistence wso2/api-manager#1664 , add an abstraction layer to separate out the token dao layer access. This way, we could plug custom dao access logic to facilitate the token persistence-related feature
• In this pr, an interface OAuth2RevocationProcessor.java is introduced and the existing implementation parts are moved to the default class DefaultOAuth2RevocationProcessor.java

When should this PR be merged

Any time

Follow up actions

• None

Checklist (for reviewing)

General

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