Parallelize cluster setup in arbitrary config tests

Cluster setup time is significant in arbitrary configs. We can
parallelize this a bit more.

Runtime of the following command decreases from ~25 seconds to ~22
seconds on my machine with this change:

```
make -C src/test/regress/ check-arbitrary-base CONFIGS=CitusDefaultClusterConfig EXTRA_TESTS=prepared_statements_1
```

src/test/regress/citus_tests/common.py

@@ -3,6 +3,7 @@ import shutil
 import sys
 import subprocess
 import atexit
+import concurrent.futures
 
 import utils
 from utils import USER, cd
@@ -24,9 +25,19 @@ def initialize_temp_dir_if_not_exists(temp_dir):
     os.chmod(temp_dir, 0o777)
 
 
+def parallel_run(function, items, *args, **kwargs):
+    with concurrent.futures.ThreadPoolExecutor() as executor:
+        futures = [
+            executor.submit(function, item, *args, **kwargs)
+            for item in items
+        ]
+        for future in futures:
+            future.result()
+
 def initialize_db_for_cluster(pg_path, rel_data_path, settings, node_names):
     subprocess.run(["mkdir", rel_data_path], check=True)
-    for node_name in node_names:
+
+    def initialize(node_name):
         abs_data_path = os.path.abspath(os.path.join(rel_data_path, node_name))
         command = [
             os.path.join(pg_path, "initdb"),
@@ -38,6 +49,8 @@ def initialize_db_for_cluster(pg_path, rel_data_path, settings, node_names):
         subprocess.run(command, check=True)
         add_settings(abs_data_path, settings)
 
+    parallel_run(initialize, node_names)
+
 
 def add_settings(abs_data_path, settings):
     conf_path = os.path.join(abs_data_path, "postgresql.conf")
@@ -49,8 +62,8 @@ def add_settings(abs_data_path, settings):
             conf_file.write(setting)
 
 
-def create_role(pg_path, port, node_ports, user_name):
-    for port in node_ports:
+def create_role(pg_path, coordinator_port, node_ports, user_name):
+    def create(port):
         command = "SELECT worker_create_or_alter_role('{}', 'CREATE ROLE {} WITH LOGIN CREATEROLE CREATEDB;', NULL)".format(
             user_name, user_name
         )
@@ -58,6 +71,8 @@ def create_role(pg_path, port, node_ports, user_name):
         command = "GRANT CREATE ON DATABASE postgres to {}".format(user_name)
         utils.psql(pg_path, port, command)
 
+    parallel_run(create, node_ports)
+
 
 def coordinator_should_haveshards(pg_path, port):
     command = "SELECT citus_set_node_property('localhost', {}, 'shouldhaveshards', true)".format(
@@ -67,7 +82,7 @@ def coordinator_should_haveshards(pg_path, port):
 
 
 def start_databases(pg_path, rel_data_path, node_name_to_ports, logfile_prefix, env_variables):
-    for node_name in node_name_to_ports.keys():
+    def start(node_name):
         abs_data_path = os.path.abspath(os.path.join(rel_data_path, node_name))
         node_port = node_name_to_ports[node_name]
         command = [
@@ -89,6 +104,11 @@ def start_databases(pg_path, rel_data_path, node_name_to_ports, logfile_prefix,
 
         subprocess.run(command, check=True)
 
+    parallel_run(start, node_name_to_ports.keys())
+
+    # We don't want parallel shutdown, here because that will fail when it's
+    # tried in this atexit call with an error like:
+    # cannot schedule new futures after interpreter shutdown
     atexit.register(
         stop_databases,
         pg_path,
@@ -96,13 +116,16 @@ def start_databases(pg_path, rel_data_path, node_name_to_ports, logfile_prefix,
         node_name_to_ports,
         logfile_prefix,
         no_output=True,
+        parallel=False,
     )
 
 
 def create_citus_extension(pg_path, node_ports):
-    for port in node_ports:
+    def create(port):
         utils.psql(pg_path, port, "CREATE EXTENSION citus;")
 
+    parallel_run(create, node_ports)
+
 
 def run_pg_regress(pg_path, pg_srcdir, port, schedule):
     should_exit = True
@@ -215,9 +238,9 @@ def logfile_name(logfile_prefix, node_name):
 
 
 def stop_databases(
-    pg_path, rel_data_path, node_name_to_ports, logfile_prefix, no_output=False
+    pg_path, rel_data_path, node_name_to_ports, logfile_prefix, no_output=False, parallel=True
 ):
-    for node_name in node_name_to_ports.keys():
+    def stop(node_name):
         abs_data_path = os.path.abspath(os.path.join(rel_data_path, node_name))
         node_port = node_name_to_ports[node_name]
         command = [
@@ -239,6 +262,12 @@ def stop_databases(
         else:
             subprocess.call(command)
 
+    if parallel:
+        parallel_run(stop, node_name_to_ports.keys())
+    else:
+        for node_name in node_name_to_ports.keys():
+            stop(node_name)
+
 
 def initialize_citus_cluster(bindir, datadir, settings, config):
     # In case there was a leftover from previous runs, stop the databases