Add helper func MessageServer.check_type_str()

Convenience function to check request type and throw
MessageError.INVALID_ARGUMENTS.

Example updated.

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Prevent MessageClient.send_message() from blocking

Prevent MessageClient.send_message() from blocking main loop. It uses
new MessageClient.send_message_async() method.

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>