We have three things that are configurable from `requests` connection pooling perspective:

* `pool_connections` - Number of urllib3 connection pools to cache before discarding the least recently used pool. Python requests default value is 10. This PR increases it to 20.
* `pool_maxsize` - The maximum number of connections to save in the pool. Improves performance in multithreaded situations. For now, we're setting it to the same value as connection_pool_size.
* `pool_block` - If pool_block is False, then more connections will are created, but not saved after the first use. Block when no free connections are available. urllib3 ensures that no more than pool_maxsize connections are used at a time. Prevents platform from flooding. By default, requests library doesn't block. We start with blocking.

This PR introduces `connection_pool_size` configuration property, which sets `pool_connections` and `pool_maxsize` to the same value. If not set, it uses 20 instead of default 10.