Fix the slow retrieval attack issue

slow retrieval attack test:

(tuf)yuyuzheng@ubuntu:~/Documents/tuf/tuf/tests/system_tests$ python test_slow_retrieval_attack.py
Slow server is active on port: 34911 ...
Communication with slow server aborted. Terminate the slow server.

mode_1	Slow Retrieval Attack was Successful!


Slow server is active on port: 34776 ...
Communication with slow server aborted. Terminate the slow server.

mode_2	Slow Retrieval Attack was Successful!


Slow server is active on port: 42898 ...
TUF ...
Download exits with timed out! Successfully avoid slow retrieval attack!


Communication with slow server aborted. Terminate the slow server.

Slow server is active on port: 43668 ...
TUF ...
Download exits with Unable to update 'timestamp.txt'.! Successfully avoid slow retrieval attack!


Communication with slow server aborted. Terminate the slow server.