Greenlet failing on OSX and Windows

```
[2017-10-16 14:50:09,571] ERROR Site:... sands.git/packed-refs fileGet error: [Errno 2] No such file or directory: u'/Users/Manwe/Tools/ZeroNet/data/1RxTdD54fbEdVZs2hZCUBWirJf5bSsAUg/sands.git/packed-refs'
[2017-10-16 14:50:09,686] DEBUG Site:... Websocket send error: ConcurrentObjectUseError: This socket is already used by another greenlet: > in UiWebsocket.py line 197 > websocket.py line 349 > websocket.py line 333 > _socket2.pyo line 419 > _socket2.pyo line 355 > _socket2.pyo line 328 > _socket2.pyo line 176
[2017-10-16 14:50:09,686] DEBUG Site:... Websocket send error: ConcurrentObjectUseError: This socket is already used by another greenlet: > in UiWebsocket.py line 197 > websocket.py line 349 > websocket.py line 333 > _socket2.pyo line 419 > _socket2.pyo line 355 > _socket2.pyo line 328 > _socket2.pyo line 176
[2017-10-16 14:50:09,686] DEBUG Site:... Websocket send error: ConcurrentObjectUseError: This socket is already used by another greenlet: > in UiWebsocket.py line 197 > websocket.py line 349 > websocket.py line 333 > _socket2.pyo line 419 > _socket2.pyo line 355 > _socket2.pyo line 328 > _socket2.pyo line 176
```

...and a lot more messages. My site is trying to read many files at once, can it be related? This fails on OSX and works on Windows.

Start with the logged call path through UiWebsocket.py line 197 and websocket.py lines 349 and 333, then compare the OSX and Windows behavior around concurrent file reads. Reproduce the site's many-file read scenario and determine what change would prevent the ConcurrentObjectUseError without breaking websocket sends.

Written by the indexing model from the issue text.