Deadlock when running master_run_on_worker in a transaction

As it opens its own connections without checking for modifications, master_run_on_worker can easily create a deadlock if it runs in a transaction block after a modification.

```
CREATE TABLE observations (tenant_id int, sensor_id int, value float);
SELECT create_distributed_table('observations', 'tenant_id');
BEGIN;
ALTER TABLE observations ADD COLUMN measurement_time timestamp default now();
SELECT master_run_on_worker(ARRAY['localhost'], ARRAY[9700], ARRAY['SELECT tenant_id FROM observations_102008']);
```