Unity exits error after a few hours running of simulator

Hi @ekolve @Lucaweihs @mattdeitke

Thanks for your great work on this simulator!

I use it for RL training and run it on a remote Ubuntu server without a physical monitor. It works fine with the `startx()` as virtual display buffer. However, when I run 4 processes at the same time in 4 different terminals, it runs good for a few hours, however, finally, it will crash in some random sense and give the following error:

`Unity exit error, please check Player.log, the last action is xxxx` and the error happens at the code line related to `fifo_server()` about receiving messages.

And when I check the Player.log, at the bottom line, I see an error `[XIO: fatal IO error 11 (Resource temporarily unavailable) on X server ":0" after 1235 requests (466 known processed) with 0 events remaining]`.

I also noticed that when everything works good (not crashed yet), from the `htop` result, it creates far more than 4 subprocesses named `/.ai2thor/release/thor-linux64-xxxxxxx`.

Anyone has encountered similar questions? Is there any solution for this. This is quite annoying after you run a program for a long time but not finished yet, and everything crashed and you have to start over again.

Thank you!
Xubo