Only read TLS CA once (#281)

## What is the goal of this PR?

We modify the cluster server client so that it only reads the TLS root CA once (if provided), and reuses the SSL credentials when prompted for a new channel.

## What are the changes implemented in this PR?

Drive-by: don't crash trying to access auth token before the stub is created.

Closes #280.
Closes #273.

Author: dmitrii-ubskii

typedb/connection/cluster/server_client.py

@@ -33,6 +33,11 @@ class _ClusterServerClient(_TypeDBClientImpl):
     def __init__(self, address: str, credential: TypeDBCredential, parallelisation: int = 2):
         super(_ClusterServerClient, self).__init__(address, parallelisation)
         self._credential = credential
+        if self._credential.tls_root_ca_path() is not None:
+            with open(self._credential.tls_root_ca_path(), 'rb') as root_ca:
+                self._channel_credentials = grpc.ssl_channel_credentials(root_ca.read())
+        else:
+            self._channel_credentials = grpc.ssl_channel_credentials()
         self._channel, self._stub = self.new_channel_and_stub()
         self._databases = _TypeDBDatabaseManagerImpl(self.stub())
 
@@ -50,14 +55,12 @@ def new_channel_and_stub(self) -> (grpc.Channel, _ClusterServerStub):
         return channel, _ClusterServerStub(channel, self._credential)
 
     def _new_channel(self) -> grpc.Channel:
-        if self._credential.tls_root_ca_path() is not None:
-            with open(self._credential.tls_root_ca_path(), 'rb') as root_ca:
-                channel_credentials = grpc.ssl_channel_credentials(root_ca.read())
-        else:
-            channel_credentials = grpc.ssl_channel_credentials()
         combined_credentials = grpc.composite_channel_credentials(
-            channel_credentials,
-            grpc.metadata_call_credentials(_CredentialAuth(credential=self._credential, token_fn=lambda: self._stub.token()))
+            self._channel_credentials,
+            grpc.metadata_call_credentials(_CredentialAuth(
+                credential=self._credential,
+                token_fn=lambda: None if self._stub is None else self._stub.token()
+            ))
         )
         return grpc.secure_channel(self._address, combined_credentials)