Improve cache size check to be more performant

Summing the sizes of each cached file every time is very inefficient.
Instead we can simply store the cache size in an constant and increase
it everytime a file is added into the cache.

src/invidious/http_server/static_assets_handler.cr

@@ -20,6 +20,7 @@ module Invidious::HttpServer
     end
 
     CACHE_LIMIT = 5_000_000 # 5MB
+    @@current_cache_size = 0
     @@cached_files = {} of Path => CachedFile
 
     # Returns metadata for the requested file
@@ -71,9 +72,8 @@ module Invidious::HttpServer
 
     # Writes file data to the cache
     private def flush_io_to_cache(io, file_path, file_info)
-      if @@cached_files.sum(&.[1].size) + file_info.size < CACHE_LIMIT
-        data_slice = io.to_slice
-        @@cached_files[file_path] = CachedFile.new(data_slice, file_info.size, file_info.modification_time)
+      if (@@current_cache_size += file_info.size) <= CACHE_LIMIT
+        @@cached_files[file_path] = CachedFile.new(io.to_slice, file_info.size, file_info.modification_time)
       end
     end
 
@@ -112,6 +112,7 @@ module Invidious::HttpServer
     #
     # This is only used in the specs to clear the cache before each handler test
     def self.clear_cache
+      @@current_cache_size = 0
       return @@cached_files.clear
     end
   end