Added connection_pool_size configuration property (preview) (#276)

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 two new configuration properties:
- `connection_pool_size` configuration property, which sets
`pool_connections`. Defaults to 20.
- `connection_pool_max_size`, which sets `pool_maxsize`. Defaults to the
same value as `connection_pool_size`.

databricks/sdk/core.py

@@ -491,6 +491,8 @@ class Config:
     metadata_service_url = ConfigAttribute(env='DATABRICKS_METADATA_SERVICE_URL',
                                            auth='metadata-service',
                                            sensitive=True)
+    max_connection_pools: int = ConfigAttribute()
+    max_connections_per_pool: int = ConfigAttribute()
 
     def __init__(self,
                  *,
@@ -893,7 +895,31 @@ def __init__(self, cfg: Config = None):
 
         self._session = requests.Session()
         self._session.auth = self._authenticate
-        self._session.mount("https://", HTTPAdapter(max_retries=retry_strategy))
+
+        # Number of urllib3 connection pools to cache before discarding the least
+        # recently used pool. Python requests default value is 10.
+        pool_connections = cfg.max_connection_pools
+        if pool_connections is None:
+            pool_connections = 20
+
+        # 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_maxsize = cfg.max_connections_per_pool
+        if cfg.max_connections_per_pool is None:
+            pool_maxsize = pool_connections
+
+        # If pool_block is False, then more connections will are created,
+        # but not saved after the first use. Blocks 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.
+        pool_block = True
+
+        http_adapter = HTTPAdapter(max_retries=retry_strategy,
+                                   pool_connections=pool_connections,
+                                   pool_maxsize=pool_maxsize,
+                                   pool_block=pool_block)
+        self._session.mount("https://", http_adapter)
 
     @property
     def account_id(self) -> str: