Allow multiple requests in parallel in Python client (#4187)

* Allow multiple requests in parallel in Python client

If you tried to do two parallel calls to the same API object in the
Python client you would get an error from urllib3 connection pool:

  Connection pool is full, discarding connection: ***

Because the default maxsize=1:
f9409436f8/urllib3/connectionpool.py (L162)

By defaulting to a higher maxsize we mitigate for the common use case
where a user is running a couple of requests in parallel.

Ideally, in the future, this should be a configuration paramater
together with the pool size.

* Add sample code after changing maxsize

modules/swagger-codegen/src/main/resources/python/rest.mustache

@@ -49,10 +49,11 @@ class RESTResponse(io.IOBase):
 
 class RESTClientObject(object):
 
-    def __init__(self, pools_size=4):
+    def __init__(self, pools_size=4, maxsize=4):
         # urllib3.PoolManager will pass all kw parameters to connectionpool
         # https://github.com/shazow/urllib3/blob/f9409436f83aeb79fbaf090181cd81b784f1b8ce/urllib3/poolmanager.py#L75
         # https://github.com/shazow/urllib3/blob/f9409436f83aeb79fbaf090181cd81b784f1b8ce/urllib3/connectionpool.py#L680
+        # maxsize is the number of requests to host that are allowed in parallel
         # ca_certs vs cert_file vs key_file
         # http://stackoverflow.com/a/23957365/2985775
 
@@ -78,6 +79,7 @@ class RESTClientObject(object):
         # https pool manager
         self.pool_manager = urllib3.PoolManager(
             num_pools=pools_size,
+            maxsize=maxsize,
             cert_reqs=cert_reqs,
             ca_certs=ca_certs,
             cert_file=cert_file,

samples/client/petstore/python/petstore_api/rest.py

@@ -58,10 +58,11 @@ class RESTResponse(io.IOBase):
 
 class RESTClientObject(object):
 
-    def __init__(self, pools_size=4):
+    def __init__(self, pools_size=4, maxsize=4):
         # urllib3.PoolManager will pass all kw parameters to connectionpool
         # https://github.com/shazow/urllib3/blob/f9409436f83aeb79fbaf090181cd81b784f1b8ce/urllib3/poolmanager.py#L75
         # https://github.com/shazow/urllib3/blob/f9409436f83aeb79fbaf090181cd81b784f1b8ce/urllib3/connectionpool.py#L680
+        # maxsize is the number of requests to host that are allowed in parallel
         # ca_certs vs cert_file vs key_file
         # http://stackoverflow.com/a/23957365/2985775
 
@@ -87,6 +88,7 @@ class RESTClientObject(object):
         # https pool manager
         self.pool_manager = urllib3.PoolManager(
             num_pools=pools_size,
+            maxsize=maxsize,
             cert_reqs=cert_reqs,
             ca_certs=ca_certs,
             cert_file=cert_file,