Add support for UNLINK [DATAREDIS-693]

**[Christoph Strobl](https://jira.spring.io/secure/ViewProfile.jspa?name=cstrobl)** opened **[DATAREDIS-693](https://jira.spring.io/browse/DATAREDIS-693?redirect=false)** and commented


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**Reference URL:** https://redis.io/commands/unlink

**Referenced from:** pull request https://github.com/spring-projects/spring-data-redis/pull/294
