feat: avoid creating unused ThreadPools (#91)

* feat: avoid creating unused ThreadPools

* docs: add requested TODOs

openapi/python-api_client.py.patch

@@ -0,0 +1,42 @@
+--- a/kubernetes/client/api_client.py
++++ b/kubernetes/client/api_client.py
+@@ -58,13 +58,15 @@ class ApiClient(object):
+         'datetime': datetime,
+         'object': object,
+     }
++    _pool = None
+ 
+-    def __init__(self, configuration=None, header_name=None, header_value=None, cookie=None):
++    def __init__(self, configuration=None, header_name=None, header_value=None,
++                 cookie=None, pool_threads=None):
+         if configuration is None:
+             configuration = Configuration()
+         self.configuration = configuration
++        self.pool_threads = pool_threads
+ 
+-        self.pool = ThreadPool()
+         self.rest_client = RESTClientObject(configuration)
+         self.default_headers = {}
+         if header_name is not None:
+@@ -74,8 +76,19 @@ class ApiClient(object):
+         self.user_agent = 'Swagger-Codegen/8.0.0-snapshot/python'
+     
+     def __del__(self):
+-        self.pool.close()
+-        self.pool.join()
++        if self._pool:
++            self._pool.close()
++            self._pool.join()
++            self._pool = None
++
++    @property
++    def pool(self):
++        """Create thread pool on first request
++         avoids instantiating unused threadpool for blocking clients.
++        """
++        if self._pool is None:
++            self._pool = ThreadPool(self.pool_threads)
++        return self._pool
+ 
+     @property
+     def user_agent(self):