make URLLIB3_POOL_SIZE configurable (#128)

Added flexibility for users to set the urllib3 connection pool size, this value can now be set in the range [10,100]

Author: sharmaharisam

src/google/appengine/runtime/default_api_stub.py

@@ -68,7 +68,16 @@
 
 URLLIB3_POOL_COUNT = 10
 
-URLLIB3_POOL_SIZE = 10
+POOL_SIZE_LOWER_LIMIT = 10
+POOL_SIZE_UPPER_LIMIT = 100
+POOL_SIZE_DEFAULT_VALUE = 10
+
+URLLIB3_POOL_SIZE = int(
+    os.environ.get('POOL_SIZE_URLLIB3', POOL_SIZE_DEFAULT_VALUE)
+)
+
+if not (POOL_SIZE_LOWER_LIMIT <= URLLIB3_POOL_SIZE <= POOL_SIZE_UPPER_LIMIT):
+  URLLIB3_POOL_SIZE = POOL_SIZE_DEFAULT_VALUE