⚡️ Optimized fetching strings from the settings cache

fix https://linear.app/ghost/issue/ENG-1105/settingscacheget-is-slow

- through profiling and flamegraphs, we can see that `_doGet` is one of
  the bottlenecks during high traffic times, sometimes taking up to 20%
  of the CPU time when hammering Ghost with `wrk`
- this is because, for the majority of settings cache lookup, we're
  running `JSON.parse`, which blocks the main thread
- whilst we're only parsing small strings, we're doing it a LOT,
  sometimes hundreds of times per request, which adds up
- this code just throws most deserializing at `JSON.parse`, so if we can
  stop it from doing that, it'd be a huge win
- my initial attempts here were to convert the _doGet function to a
  smarter deserializing, by looking up `cacheEntry.type` and acting
  accordingly
- however, it became a bit of a logical nightmare, and difficult to
  reason about for now (i still think we should do it)
- therefore, I'm just doing to add a hotpath fix to catch 99% of
  usecases, which is checking the type of the cache entry and returning
  the value if it's a string
- on a trivial benchmark locally, this causes Ghost to return 30% more
  requests per second!!

ghost/core/core/shared/settings-cache/CacheManager.js

@@ -64,6 +64,10 @@ class CacheManager {
             return cacheEntry;
         }
 
+        // TODO: I think we should be a little smarter here and deserialize the value based on the type
+        //       rather than trying to parse everything as JSON, which is very slow when we do it hundreds
+        //       of times per request.
+
         // Default behavior is to try to resolve the value and return that
         try {
             // CASE: handle literal false
@@ -71,6 +75,11 @@ class CacheManager {
                 return false;
             }
 
+            // CASE: hotpath early return for strings which are already strings
+            if (cacheEntry.type === 'string' && typeof cacheEntry.value === 'string') {
+                return cacheEntry.value || null;
+            }
+
             // CASE: if a string contains a number e.g. "1", JSON.parse will auto convert into integer
             if (!isNaN(Number(cacheEntry.value))) {
                 return cacheEntry.value || null;