Excessive memory usage

[ln5]

Possibly related to unhandled errors (#70, #81), a scan eats huge amounts of memory. Here's an example:

2018-02-06T00:04:41+0100 [INFO]: Performing a measurement scan with 6216 relays.

10h later, the python process consumes about 5GB of RAM (RSS):

F   UID   PID  PPID PRI  NI    VSZ   RSS WCHAN  STAT TTY        TIME COMMAND
0  1000   996   583  20   0 5156300 5035808 SyS_ep Sl+ pts/1    2:30 /usr/bin/python /home/bwscanner/.local/bin/bwscan --launch-tor --loglevel debug scan

[ln5]

I'm running branch nn_revert_remove_hashing_readbody since 9h now and am seeing a very different memory usage pattern. Even with the Unhandled errors (both of them mentioned above) RSS has never exceeded 360 MB and is shrinking now and then (current RSS is 272 MB and VSZ 472 MB).

[DonnchaC]

@ln5 Well spotted. We need to reintroduce the hashingReadBody code to minimize the memory usage when running the scans.