add ability to get list of all registered hashes from sessions

lib/floof/session_manager.ex

@@ -73,6 +73,10 @@ defmodule Floof.SessionManager do
     GenServer.call(__MODULE__, {:peek, key}, :infinity)
   end
 
+  def get_hashes() do
+    GenServer.call(__MODULE__, :get_hashes, :infinity)
+  end
+
   def drop(key, subkeys) do
     GenServer.cast(__MODULE__, {:drop, key, subkeys})
   end
@@ -117,6 +121,21 @@ defmodule Floof.SessionManager do
      end, state}
   end
 
+  @impl true
+  def handle_call(:get_hashes, _, state) do
+    ret =
+      case :dets.match(state.dets_file, {'_', '$2'}) do
+        {:error, e} ->
+          Logger.error("garbage collection error: #{inspect(e)}")
+          :error
+
+        keys ->
+          {:ok, Enum.map(keys, fn [x] -> x end)}
+      end
+
+    {:reply, ret, state}
+  end
+
   @impl true
   def handle_cast({:modify_subscription, key, handler}, state) do
     {_, subs2} =