Refactor ReactiveCaching for easier usage by services

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The following discussion from gitlab-ce!26840 should be addressed:

• @splattael started a discussion:

  Note: In near future, we are planning to also remove the need to have an id defined. In this iteration it's much easier to leave it unused as nil.

  @rpereira2 Should we create an issue labelled gitlab-ce~3011693 for this?

The aim of this issue is to refactor ReactiveCaching to achieve the following goals:

1. Since ReactiveCaching can now be used by services, move it to a more appropriate location than its current location.
2. Remove the need for defining an id method when ReactiveCaching is being used by a service.