That way it will be obvious that sbws isn't really going to try to
log to /nonexist and that sbws must set this option for real elsewhere.

2018-06-06 04:42:03 ERROR Thread-5 scanner.py:225 - dispatch_worker_thread - Unhandled exception in worker thread
Traceback (most recent call last):
  File "/home/pastly/src/simple-bw-scanner/sbws/core/scanner.py", line 223, in dispatch_worker_thread
    return measure_relay(*a, **kw)
  File "/home/pastly/src/simple-bw-scanner/sbws/core/scanner.py", line 138, in measure_relay
    dest = destinations.next()
  File "/home/pastly/src/simple-bw-scanner/sbws/lib/destination.py", line 222, in next
    self._perform_usability_test()
  File "/home/pastly/src/simple-bw-scanner/sbws/lib/destination.py", line 171, in _perform_usability_test
    circ_id = self._cb.build_circuit([None, exit.fingerprint])
  File "/home/pastly/src/simple-bw-scanner/sbws/lib/circuitbuilder.py", line 175, in build_circuit
    for r in path]
  File "/home/pastly/src/simple-bw-scanner/sbws/lib/circuitbuilder.py", line 175, in <listcomp>
    for r in path]
AttributeError: 'NoneType' object has no attribute 'fingerprint'
2018-06-06 04:42:03 ERROR Thread-10 scanner.py:280 - closure - Unhandled exception caught while measuring torbogen: <class 'AttributeError'> 'NoneType' object has no attribute 'fingerprint'

also:
* fix again number of header lines
* FIXME: tests for bw lines should be independent on headers,
  Otherwise they'll fail everytime we change headers.

also:
* remove latest_bandwidth outside of class, since it should be
  generated from timestamp
* pass all init arguments as string
* require timestamp argument
* fix tests with changes

they do not need to be in 3rd and 4th posstions.

Also:
* add latest_bandwidth header
* add file_created header
* fix bw files header tests
* disable tests that assume timestamp to be the file creation
time
* if we use ISO 8601 with T as separator,
  datetime format string is not needed

On one hand, for weeks now I've been running sbws and it seems to be
correctly doing downloads over tor. I've monitored existing circs and
streams with a script that queries the control port and what I saw made
sense.

On the other hand, I just tried running sbws on my laptop over a VPN
that blocks resolution of a hostname of mine. I got a proxy error with
a suspicious hostname in the message that led me to believe that
destination usability checks were not actually being done over Tor like
they are supposed to be.

The way Requests tells you to use proxies is by specifying them *not* on
the session object, but as an argument[0]. So do that.

[0]: http://docs.python-requests.org/en/master/user/advanced/#proxies

remove unused key tor_control_socket
remove duplicate key started_filepath in scanner section

GH: closes #139