Calibre: More QoL tweaks (#7545)

* Wireless: Optimize memory usage in StreamMessageQueue (use an array of string ropes, that we only concatenate once). Allowed to relax the throttling, making transfers that much faster.
* Persist: Add a "zstd" codec, that uses the "luajit" codec, but compressed via zstd. Since both of those are very fast, it pretty much trounces everything in terms of speed and size ;).
* Persist: Implemented a "writes_to_file" framework, much like the existing "reads_from_file" one. And use it in the zstd codec to avoid useless temporary string interning.
* Metadata: Switch to the zstd codec.

frontend/ui/data/onetime_migration.lua

@@ -203,7 +203,7 @@ if last_migration_date < 20210414 then
     logger.info("Performing one-time migration for 20210414")
 
     local cache_path = DataStorage:getDataDir() .. "/cache/calibre"
-    ok, err = os.remove(cache_path .. "/books.dat")
+    local ok, err = os.remove(cache_path .. "/books.dat")
     if not ok then
        logger.warn("os.remove:", err)
     end