FEATURE: enable_performance_http_headers for performance diagnostics

This adds support for DISCOURSE_ENABLE_PERFORMANCE_HTTP_HEADERS
when set to `true` this will turn on performance related headers

```text
X-Redis-Calls: 10     # number of redis calls
X-Redis-Time: 1.02    # redis time in seconds
X-Sql-Commands: 102   # number of SQL commands
X-Sql-Time: 1.02      # duration in SQL in seconds
X-Queue-Time: 1.01    # time the request sat in queue (depends on NGINX)
```

To get queue time NGINX must provide: HTTP_X_REQUEST_START

We do not recommend you enable this without thinking, it exposes information
about what your page is doing, usually you would only enable this if you
intend to strip off the headers further down the stream in a proxy

lib/middleware/request_tracker.rb

@@ -165,6 +165,20 @@ class Middleware::RequestTracker
     # possibly transferred?
     if info && (headers = result[1])
       headers["X-Runtime"] = "%0.6f" % info[:total_duration]
+
+      if GlobalSetting.enable_performance_http_headers
+        if redis = info[:redis]
+          headers["X-Redis-Calls"] = redis[:calls].to_s
+          headers["X-Redis-Time"] = "%0.6f" % redis[:duration]
+        end
+        if sql = info[:sql]
+          headers["X-Sql-Calls"] = sql[:calls].to_s
+          headers["X-Sql-Time"] = "%0.6f" % sql[:duration]
+        end
+        if queue = env['REQUEST_QUEUE_SECONDS']
+          headers["X-Queue-Time"] = "%0.6f" % queue
+        end
+      end
     end
 
     if env[Auth::DefaultCurrentUserProvider::BAD_TOKEN] && (headers = result[1])