Myth Backend handles loss of disk poorly

[danielk]

When an NFS server goes down in the middle of a recording any of the backend's ThreadedFileWriters writing to that disk become permanently wedged and CPU usage shoots up to near 100%.

Using aio_write() instead of write() should allow us to better handle this condition. We should at minimum not use 100% CPU, and ideally continue recording to other disks including with the currently blocked recorder.