Fix the race condition caused by `with_new_connections_blocked`

`with_new_connections_blocked` was introduced at #14938.
But the method sometimes causes `@new_cons_enabled = false` then never
toggled to true.

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

@@ -350,8 +350,7 @@ def initialize(spec)
         # currently in the process of independently establishing connections to the DB.
         @now_connecting = 0
 
-        # A boolean toggle that allows/disallows new connections.
-        @new_cons_enabled = true
+        @new_cons_blocks = 0
 
         @available = ConnectionLeasingQueue.new self
       end
@@ -700,13 +699,10 @@ def checkout_for_exclusive_access(checkout_timeout)
         end
 
         def with_new_connections_blocked
-          previous_value = nil
-          synchronize do
-            previous_value, @new_cons_enabled = @new_cons_enabled, false
-          end
+          synchronize { @new_cons_blocks += 1 }
           yield
         ensure
-          synchronize { @new_cons_enabled = previous_value }
+          synchronize { @new_cons_blocks -= 1 }
         end
 
         # Acquire a connection by one of 1) immediately removing one
@@ -758,7 +754,7 @@ def try_to_checkout_new_connection
           # and increment @now_connecting, to prevent overstepping this pool's @size
           # constraint
           do_checkout = synchronize do
-            if @new_cons_enabled && (@connections.size + @now_connecting) < @size
+            if @new_cons_blocks.zero? && (@connections.size + @now_connecting) < @size
               @now_connecting += 1
             end
           end