Closes #7235. Instead of setting KeepService's pycurl.TIMEOUT_MS, set pycurl.LOW_SPEE...

Closes #7235. Instead of setting KeepService's pycurl.TIMEOUT_MS, set pycurl.LOW_SPEED_LIMIT and pycurl.LOW_SPEED_TIME.
Default LOW_SPEED_LIMIT is 32768 bytes per second. Default LOW_SPEED_TIME is 64 seconds.
If the user specifies a length-two tuple, the first item sets CONNECTTIMEOUT_MS, the second item sets LOW_SPEED_TIME,
and LOW_SPEED_LIMIT is set to 32768 bytes per second.

Added bandwidth similator to keepstub, which uses millisecond precision (like curl) to measure timeouts.
Added tests to test_keep_client and modified existing tests to only use integers.