Move feature flag list into process cache

When we switched from a single-threaded application server (Unicorn) to
a multithreaded one (Puma), we did not realize that Puma often reaps
threads after a request is done and recreates them later. This makes the
thread-local cache ineffective, as the cache does not store anything
beyond the lifetime of the thread.

Since `ActiveSupport::Cache::MemoryStore` is thread-safe, we should be
able to switch the L1 cache for the feature flag list to use this to
reduce load on Redis.

Since read and write access is synchronized, this does have the side
effect of adding contention when feature flags are accessed.

We made a similar change in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/26935, and this
seems to be working fine.

Discovered in
https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/9414

changelogs/unreleased/sh-feature-flag-persisted-process-cache.yml

+---
+title: Move feature flag list into process cache
+merge_request: 27511
+author:
+type: performance

lib/feature.rb

@@ -38,7 +38,7 @@ class Feature
         begin
           # We saw on GitLab.com, this database request was called 2300
           # times/s. Let's cache it for a minute to avoid that load.
-          Gitlab::ThreadMemoryCache.cache_backend.fetch('flipper:persisted_names', expires_in: 1.minute) do
+          Gitlab::ProcessMemoryCache.cache_backend.fetch('flipper:persisted_names', expires_in: 1.minute) do
             FlipperFeature.feature_names
           end
         end

spec/lib/feature_spec.rb

@@ -42,7 +42,7 @@ describe Feature do
         .once
         .and_call_original
 
-      expect(Gitlab::ThreadMemoryCache.cache_backend)
+      expect(Gitlab::ProcessMemoryCache.cache_backend)
         .to receive(:fetch)
         .once
         .with('flipper:persisted_names', expires_in: 1.minute)