Draft: Add new repositories set cache class

Hunter Stewartrequested to merge
506430-new-cache into master

Why

Gitlab::RepositorySetCache#fetch is not atomic which can result in stale reads. See #506430 for full details

What

Add new class Gitlab::Repositories::SetCache

The new class is similar in behavior to Gitlab::RepositorySetCache but introduces some new things:

  • The class is moved into the correct bounded context.
  • The inheritance is collapsed.
  • Lua scripts handle the write and fetch behavior so that it can be done atomically.
  • Hash tags are introduced since multi key operations via the Lua script would otherwise raise a RedisClient::CommandError: CROSSSLOT error.

This MR does not invoke the new class but only adds it and its spec.

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Related to #506430

Edited by Hunter Stewart

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