Add stub cache

WIP - probably should be generalized though. Want to think
about a library for that.

src/machi_merkle_cache.erl

@@ -0,0 +1,86 @@
+-module(machi_merkle_cache).
+-behaviour(gen_server).
+
+-export([
+         child_spec/1,
+         start_link/1,
+         get/2,
+         put/2
+        ]).
+
+%% gen server callbacks
+-export([
+         init/1,
+         handle_cast/2,
+         handle_call/3,
+         handle_info/2,
+         terminate/2,
+         code_change/3
+        ]).
+
+-define(EXPIRE_TIMER, 60*1000).
+
+child_spec(_FLU) ->
+    {}.
+
+start_link(FLU) ->
+    gen_server:start_link({local, make_cache_name(FLU)}, ?MODULE, [FLU], []).
+
+get(_FLU, _File) ->
+    %% case ets:lookup(make_cache_name_ets(FLU), File) of
+    %%     undefined -> undefined;
+    %%     [MT] -> {ok, MT}
+    %% end.
+    undefined.
+
+put(_FLU, _MT) ->
+    %% gen_server:cast(make_cache_name(FLU), {put, MT}).
+    ok.
+
+%% gen server callbacks
+init([FLU]) ->
+    Tid = ets:new(make_cache_name_ets(FLU), [named_table, {keypos,2}, 
+                                             {read_concurrency, true}, 
+                                             {write_concurrency, true}]),
+    schedule_expire_tick(),
+    {ok, Tid}.
+
+handle_cast({put, MT}, Tid) ->
+    ets:insert(Tid, MT),
+    {noreply, Tid};
+handle_cast(Msg, S) ->
+    lager:warning("unknown cast ~p", [Msg]),
+    {noreply, S}.
+
+handle_call(Msg, From, S) ->
+    lager:warning("unknown call ~p from ~p", [Msg, From]),
+    {reply, whaaaaa, S}.
+
+handle_info(merkle_expire_tick, Tid) ->
+    do_expire(Tid),
+    schedule_expire_tick(),
+    {noreply, Tid};
+handle_info(Msg, S) ->
+    lager:warning("unknown info message ~p", [Msg]),
+    {noreply, S}.
+
+terminate(Reason, Tid) ->
+    lager:debug("Terminating merkle cache ETS table ~p because ~p",
+                [Tid, Reason]),
+    ok.
+
+code_change(_, _, S) ->
+    {ok, S}.
+
+%% private
+
+schedule_expire_tick() ->
+    erlang:send_after(?EXPIRE_TIMER, self(), merkle_expire_tick).
+
+make_cache_name_ets(_FLU) ->
+    merkle_ets_cache.
+
+make_cache_name(_FLU) ->
+    merkle_cache.
+
+do_expire(_Tid) -> ok.