Give each redshift client their own boto session

since the boto session is not thread-safe, using the default session in a multi-threaded scenario will result in concurrency errors

plugins/redshift/dbt/adapters/redshift/connections.py

@@ -99,7 +99,8 @@ def fetch_cluster_credentials(cls, db_user, db_name, cluster_id,
  must already exist in the database, or else an error will occur"""
 
  if iam_profile is None:
- boto_client = boto3.client('redshift')
+ session = boto3.Session()
+ boto_client = session.client("redshift")
  else:
  logger.debug("Connecting to Redshift using 'IAM'" +
  f"with profile {iam_profile}")