Introduce wait_for_subscription_sync for TAP tests

Hi,

In tap tests for logical replication, we have the following code in many places:

$node_publisher->wait_for_catchup('tap_sub');
my $synced_query =
  "SELECT count(1) = 0 FROM pg_subscription_rel WHERE srsubstate NOT
IN ('r', 's');";
$node_subscriber->poll_query_until('postgres', $synced_query)
  or die "Timed out while waiting for subscriber to synchronize data";

Also, we sometime forgot to check either one, like we fixed in commit
1f50918a6fb02207d151e7cb4aae4c36de9d827c.

I think we can have a new function to wait for all subscriptions to
synchronize data. The attached patch introduce a new function
wait_for_subscription_sync(). With this function, we can replace the
above code with this one function as follows:

$node_subscriber->wait_for_subscription_sync($node_publisher, 'tap_sub');

Regards,

-- 
Masahiko Sawada
EDB:  https://www.enterprisedb.com/