Concurrent removal of a worker node and a reference table drop fails

Setup consists of 4 workers and a reference table.

- Connection 1:
-- Start transaction
-- Remove a worker node, don’t commit
- Connection 2
-- Drop a reference table (gets blocked)
- Connection 1:
-- commit
- Connection 2:
-- Unblocked, fails with an error

```
ERROR: tuple concurrently updated
CONTEXT: SQL statement "SELECT master_drop_all_shards(v_obj.objid, v_obj.schema_name, v_obj.object_name)"
PL/pgSQL function citus_drop_trigger() line 21 at PERFORM
```