hopefully fixes lingering ydl concurrency issue

which was causing awfulness like this:
2018-09-30 04:39:54,410 19101 INFO BrozzlingThread:58486 brozzler.ydl._build_youtube_dl.<locals>._YoutubeDL._push_stitched_up_vid_to_warcprox(ydl.py:164) pushing '0 - unknown' video stitched-up as application/octet-stream (228243844 bytes) to warcprox at wbgrp-svc408.us.archive.org:8000 with url youtube-dl:00001:https://www.facebook.com/CongresswomanRosaDeLauro/
2018-09-30 04:39:58,092 19101 INFO BrozzlingThread:58486 brozzler.ydl._build_youtube_dl.<locals>._YoutubeDL._push_stitched_up_vid_to_warcprox(ydl.py:164) pushing '0 - 1080x607' video stitched-up as video/mp4 (228243844 bytes) to warcprox at wbgrp-svc045.us.archive.org:8001 with url youtube-dl:00037:https://instagram.com/p/BfJvqhfnQ0C/
2018-09-30 04:40:05,120 19101 INFO BrozzlingThread:58486 brozzler.ydl._build_youtube_dl.<locals>._YoutubeDL._push_stitched_up_vid_to_warcprox(ydl.py:164) pushing '0 - unknown' video stitched-up as application/octet-stream (228243844 bytes) to warcprox at wbgrp-svc107.us.archive.org:8000 with url youtube-dl:00009:https://www.facebook.com/LDS
2018-09-30 04:40:09,450 19101 INFO BrozzlingThread:58486 brozzler.ydl._build_youtube_dl.<locals>._YoutubeDL._push_stitched_up_vid_to_warcprox(ydl.py:164) pushing '22 - 1280x720 (hd720)' video stitched-up as video/mp4 (228243844 bytes) to warcprox at wbgrp-svc407.us.archive.org:8000 with url youtube-dl:00048:https://www.youtube.com/watch?v=-gH28zrMmAM
2018-09-30 04:40:14,327 19101 INFO BrozzlingThread:58486 brozzler.ydl._build_youtube_dl.<locals>._YoutubeDL._push_stitched_up_vid_to_warcprox(ydl.py:164) pushing 'hls-2176 - 1280x720' video stitched-up as video/mp4 (228243844 bytes) to warcprox at wbgrp-svc108.us.archive.org:8000 with url youtube-dl:00001:https://twitter.com/RepTedLieu/status/1010212963897233408
2018-09-30 04:40:23,018 19101 INFO BrozzlingThread:58486 brozzler.ydl._build_youtube_dl.<locals>._YoutubeDL._push_stitched_up_vid_to_warcprox(ydl.py:164) pushing '0 - unknown' video stitched-up as application/octet-stream (228243844 bytes) to warcprox at wbgrp-svc048.us.archive.org:8001 with url youtube-dl:00005:https://www.facebook.com/SenDuckworth/
2018-09-30 04:40:29,553 19101 INFO BrozzlingThread:58486 brozzler.ydl._build_youtube_dl.<locals>._YoutubeDL._push_stitched_up_vid_to_warcprox(ydl.py:164) pushing '0 - unknown' video stitched-up as application/octet-stream (228243844 bytes) to warcprox at wbgrp-svc045.us.archive.org:8000 with url youtube-dl:00009:http://www.facebook.com/repkathleenrice
2018-09-30 04:40:37,057 19101 INFO BrozzlingThread:58486 brozzler.ydl._build_youtube_dl.<locals>._YoutubeDL._push_stitched_up_vid_to_warcprox(ydl.py:164) pushing '22 - 1280x720 (hd720)' video stitched-up as video/mp4 (228243844 bytes) to warcprox at wbgrp-svc406.us.archive.org:8000 with url youtube-dl:00023:https://www.youtube.com/watch?v=MaamqVF87mE
2018-09-30 04:40:41,298 19101 INFO BrozzlingThread:58486 brozzler.ydl._build_youtube_dl.<locals>._YoutubeDL._push_stitched_up_vid_to_warcprox(ydl.py:164) pushing '22 - 1280x720 (hd720)' video stitched-up as video/mp4 (228243844 bytes) to warcprox at wbgrp-svc403.us.archive.org:8000 with url youtube-dl:00039:https://www.youtube.com/watch?v=pRpMp4H8El0
2018-09-30 04:40:45,613 19101 INFO BrozzlingThread:58486 brozzler.ydl._build_youtube_dl.<locals>._YoutubeDL._push_stitched_up_vid_to_warcprox(ydl.py:164) pushing 'hls-2176 - 1280x720' video stitched-up as video/mp4 (228243844 bytes) to warcprox at wbgrp-svc408.us.archive.org:8000 with url youtube-dl:00001:https://twitter.com/RepKevinCramer/status/999771072206639104
i.e. pushing the same stitched-up video to a bunch of wrong places :(

setup.py

@@ -32,7 +32,7 @@ def find_package_data(package):
 
 setuptools.setup(
         name='brozzler',
-        version='1.5.dev307',
+        version='1.5.dev308',
         description='Distributed web crawling with browsers',
         url='https://github.com/internetarchive/brozzler',
         author='Noah Levitt',