PYTHON-2776 Disable writes and other unsupported operations in snapshot reads (#660)

Rely on the server to report an error for unsupported snapshot read
operations by sending readConcern with all commands, even writes.

(cherry picked from commit fd84565)

Author: ShaneHarvey

pymongo/client_session.py

@@ -869,6 +869,9 @@ def _txn_read_preference(self):
     def _apply_to(self, command, is_retryable, read_preference, sock_info):
         self._check_ended()
 
+        if self.options.snapshot:
+            self._update_read_concern(command, sock_info)
+
         self._server_session.last_use = monotonic.time()
         command['lsid'] = self._server_session.session_id