use cache for estimated transaction count

apps/explorer/lib/explorer/application.ex

@@ -8,6 +8,7 @@ defmodule Explorer.Application do
   alias Explorer.Admin
   alias Explorer.Chain.BlockNumberCache
   alias Explorer.Repo.PrometheusLogger
+  alias Explorer.Chain.TransactionCountCache
 
   @impl Application
   def start(_type, _args) do
@@ -27,7 +28,8 @@ defmodule Explorer.Application do
       Supervisor.child_spec({Task.Supervisor, name: Explorer.MarketTaskSupervisor}, id: Explorer.MarketTaskSupervisor),
       Supervisor.child_spec({Task.Supervisor, name: Explorer.TaskSupervisor}, id: Explorer.TaskSupervisor),
       {Registry, keys: :duplicate, name: Registry.ChainEvents, id: Registry.ChainEvents},
-      {Admin.Recovery, [[], [name: Admin.Recovery]]}
+      {Admin.Recovery, [[], [name: Admin.Recovery]]},
+      {TransactionCountCache, [[], []]}
     ]
 
     children = base_children ++ configurable_children()

apps/explorer/lib/explorer/chain.ex

@@ -20,7 +20,6 @@ defmodule Explorer.Chain do
 
   import EthereumJSONRPC, only: [integer_to_quantity: 1]
 
-  alias Ecto.Adapters.SQL
   alias Ecto.{Changeset, Multi}
 
   alias Explorer.Chain.{
@@ -39,6 +38,7 @@ defmodule Explorer.Chain do
     Token,
     TokenTransfer,
     Transaction,
+    TransactionCountCache,
     Wei
   }
 
@@ -1842,10 +1842,7 @@ defmodule Explorer.Chain do
   """
   @spec transaction_estimated_count() :: non_neg_integer()
   def transaction_estimated_count do
-    %Postgrex.Result{rows: [[rows]]} =
-      SQL.query!(Repo, "SELECT reltuples::BIGINT AS estimate FROM pg_class WHERE relname='transactions'")
-
-    rows
+    TransactionCountCache.value()
   end
 
   @doc """