[wip] websockets should have their own supervision tree

I also hate this bullshit of

- fd_client having such a stupid name
- websocket lib needs some love

basically I think this should be the approach:

renames:

- fd_clients    -> fd_http
- fd_client_man -> fd_http_client_man
- fd_client_sup -> fd_http_client_sup
- fd_client     -> fd_http_client
- fd_cache      -> fd_wfc_cache

new trees:

- tetris        -> new tree
- websockets    -> probably should fall into http supervision tree

Notes:
- fd_client_man is necessary because someone needs to own the listen socket
- rewrite fd_client as gen_server

src/fd_tetris.erl

@@ -2,7 +2,11 @@
 %
 % manages state for a single game of tetris
 %
-% https://www.erlang.org/docs/24/man/gen_server
+% sends parent process messages `{tetris, String}` where String is an encoded
+% JSON blob meant to be sent to the page script in /priv/static/js/ts/tetris.ts
+%
+% Refs:
+% 1. https://www.erlang.org/docs/24/man/gen_server
 -module(fd_tetris).
 
 -behavior(gen_server).
@@ -27,6 +31,14 @@
 %% caller context below this line
 %%-----------------------------------------------------------------------------
 
+-spec ws_msg(Tetris, Message) -> ok
+    when Tetris  :: pid(),
+         Message :: fd_ws:ws_msg().
+
+ws_msg(Tetris, Msg) ->
+    gen_server:cast(Tetris, {ws_msg, Msg}).
+
+
 -spec start_link() -> {ok, pid()} | {error, term()}.
 start_link() ->
     gen_server:start_link(?MODULE, [self()], []).