Allow offline fetching for the ChainStatistics server

lib/explorer/application.ex

@@ -26,7 +26,11 @@ defmodule Explorer.Application do
   defp children(_) do
     import Supervisor.Spec
     exq_options = [] |> Keyword.put(:mode, :enqueuer)
-    [supervisor(Exq, [exq_options]) | children()]
+    [
+      supervisor(Exq, [exq_options]),
+      worker(Explorer.Servers.ChainStatistics, [])
+      | children()
+    ]
   end
 
   defp children do
@@ -34,7 +38,6 @@ defmodule Explorer.Application do
     [
       supervisor(Explorer.Repo, []),
       supervisor(ExplorerWeb.Endpoint, []),
-      worker(Explorer.Servers.ChainStatistics, [])
     ]
   end
 end

lib/explorer/servers/chain_statistics.ex

@@ -7,7 +7,12 @@ defmodule Explorer.Servers.ChainStatistics do
 
   @interval 1_000
 
-  def fetch, do: GenServer.call(__MODULE__, :fetch)
+  def fetch do
+    case GenServer.whereis(__MODULE__) do
+      nil -> Chain.fetch()
+      pid -> GenServer.call(pid, :fetch)
+    end
+  end
   def start_link, do: start_link(%Chain{})
   def start_link(%Chain{} = chain) do
     GenServer.start_link(__MODULE__, chain, name: __MODULE__)