Rewrite client media download and support hash-based remote download

Move most of the media-related code in client.cpp into a new class
ClientMediaDownloader (clientmedia.cpp, clientmedia.h). Among other
things, this class does the following things:

- Download [remote_server][sha1] instead of [remote_server][name]. This
is to support servers that provide the same file name with different
contents.
- Initially fetch [remote_server]index.mth. This file should follow the
Minetest Hashset format (currently version 1) and contain a list of SHA1
hashes that exist on the server.
- The list of needed SHA1s is uploaded (via HTTP POST) when index.mth is
requested, so servers can optionally narrow down the list to the needs
of the client.
- If index.mth is missing (HTTP response code 404), we enter compat mode,
fetching [remote_server][name] as before this commit.
- remote_server can now contain multiple servers, separated by commas.
The downloader code attempts to split requests between the different
servers, as permitted by each server's index.mth. If one server claims
to have a file but actually doesn't (or something fails), we ask a
different server that also claims to have it.
- As before, when none of the remote servers provide a particular
file, we download it via the conventional method, i.e. using
the minetest protocol: TOSERVER_REQUEST_MEDIA / TOCLIENT_MEDIA.
- Bugfix: Every downloaded file's SHA1 is now verified against the SHA1
announced by the minetest server (before loading it and inserting it
into the file cache).
- Bugfix: Only send TOSERVER_RECEIVED_MEDIA when we actually have all
media. This should fix #863.

minetest.conf.example

@@ -191,9 +191,16 @@
 # right clicks when holding the right mouse button
 #repeat_rightclick_time = 0.25
 
-# will only work for servers which use remote_media setting
-# and only for clients compiled with cURL
-#media_fetch_threads = 8
+# Default timeout for cURL, in milliseconds
+# Only has an effect if compiled with cURL
+#curl_timeout = 5000
+# Limits number of parallel HTTP requests. Affects:
+# - Media fetch if server uses remote_media setting
+# - Serverlist download and server announcement
+# - Downloads performed by main menu (e.g. mod manager)
+# - Downloads performed by mods (minetest.httpfetch)
+# Only has an effect if compiled with cURL
+#curl_parallel_limit = 8
 
 # Url to the server list displayed in the Multiplayer Tab
 #serverlist_url = servers.minetest.net