NUM_THREADS should match max conns in http conn pool

We're using the default value of requests/urllib3 for the
max size of a connection pool, which is 10.  We should match
this value so that we don't cycle through opening/closing connections.

This should have no functional changes.  We're just making less
http connections overall.

https://github.com/boto/botocore/blob/develop/botocore/vendored/requests/adapters.py#L31