Clear available connections immediately

It'll be re-cleared when it's rebuilt in with_new_connections_blocked's
ensure, but we still need to clear it inside this synchronize -- we've
disconnected connections that may be available in the queue, and while
other threads are not allowed to make *new* connections, they are still
allowed to take existing ones from there.

This was incorrectly removed in d314646c.

activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb

@@ -422,6 +422,7 @@ def disconnect(raise_on_acquisition_timeout = true)
               conn.disconnect!
             end
             @connections = []
+            @available.clear
           end
         end
       end
@@ -454,6 +455,7 @@ def clear_reloadable_connections(raise_on_acquisition_timeout = true)
               conn.disconnect! if conn.requires_reloading?
             end
             @connections.delete_if(&:requires_reloading?)
+            @available.clear
           end
         end
       end