Connection Pool: fix LRFU implementation to not actually be LRU

The numbers of the native time unit were so small the CRF was always 1,
making it an LRU. This commit switches the time to miliseconds and changes
the time delta multiplier to the one yielding mostly highest hit rates according
to the paper

lib/pleroma/gun/connection_pool/worker.ex

@@ -12,7 +12,7 @@ def start_link([key | _] = opts) do
   def init([key, uri, opts, client_pid]) do
     with {:ok, conn_pid} <- Gun.Conn.open(uri, opts),
          Process.link(conn_pid) do
-      time = :erlang.monotonic_time()
+      time = :erlang.monotonic_time(:millisecond)
 
       {_, _} =
         Registry.update_value(@registry, key, fn _ ->
@@ -31,7 +31,7 @@ def init([key, uri, opts, client_pid]) do
 
   @impl true
   def handle_cast({:add_client, client_pid, send_pid_back}, %{key: key} = state) do
-    time = :erlang.monotonic_time()
+    time = :erlang.monotonic_time(:millisecond)
 
     {{conn_pid, _, _, _}, _} =
       Registry.update_value(@registry, key, fn {conn_pid, used_by, crf, last_reference} ->
@@ -116,6 +116,6 @@ def handle_info({:DOWN, _ref, :process, pid, reason}, state) do
 
   # LRFU policy: https://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.55.1478
   defp crf(time_delta, prev_crf) do
-    1 + :math.pow(0.5, time_delta / 100) * prev_crf
+    1 + :math.pow(0.5, 0.0001 * time_delta) * prev_crf
   end
 end