Synchronize get value loaders taking cache key into account

[miensol]

The previous version synchronize all calls to get(key, valueLoader). After this PR, the calls to value loader will only be synchronized if we do not have a value for the given key.

This is an extension of #2082.

This version synchronizes loaders taking key into account.

Fixes #2079

• You have read the Spring Data contribution guidelines.
• You use the code formatters provided here and have them applied to your changes. Don’t submit any formatting related changes.
• You submit test cases (unit or integration tests) that back your changes.
• You added yourself as author in the headers of the classes you touched. Amend the date range in the Apache license header if needed. For new types, add the license header (copy from another file and set the current year only).

[christophstrobl]

I share the concerns, about keeping a mutex within RedisCache, @mp911de outlined in #2079, .
At this point this is something that we don’t want to add.
Thanks for the time invested, both, on discussing the issue, as well as creating this PR.