Cache shard name integer check

We only need to verify that all of the shard names are integers _once_,
otherwise we will perform an expensive integer check on every shard
name, every time we call `shard_names`.

lib/active_record_shards/connection_switcher.rb

@@ -142,9 +142,7 @@ def current_shard_id
     end
 
     def shard_names
-      unless config_for_env.fetch(SHARD_NAMES_CONFIG_KEY, []).all? { |shard_name| shard_name.is_a?(Integer) }
-        raise "All shard names must be integers: #{config_for_env[SHARD_NAMES_CONFIG_KEY].inspect}."
-      end
+      all_shard_names_are_integers?
 
       config_for_env[SHARD_NAMES_CONFIG_KEY] || []
     end
@@ -171,6 +169,16 @@ def config_for_env
     end
     alias_method :check_config_for_env, :config_for_env
 
+    def all_shard_names_are_integers?
+      @all_shard_names_are_integers ||= begin
+        if config_for_env.fetch(SHARD_NAMES_CONFIG_KEY, []).all? { |shard_name| shard_name.is_a?(Integer) }
+          true
+        else
+          raise "All shard names must be integers: #{config_for_env[SHARD_NAMES_CONFIG_KEY].inspect}."
+        end
+      end
+    end
+
     def switch_connection(options)
       ensure_legacy_connection_handling if ActiveRecord.version >= Gem::Version.new('6.1')