Prevent multiple jobs running concurrently

Created by: cshields

Previously, for some unknown reason when the observation task in the client started, the client would attempt 2 instances of rtl_fm and sometimes 2 instances of oggenc. In the case of a second instance of rtl_fm it dies out quickly because the USB device is already taken. In the case of a second oggenc, it will result in a file with 0 bites overwriting the file with the actual data, which the client will stash in /data/incomplete/.

I do not know why appscheduler starts 2 instances, I've checked the jobs db and there is truly only one job per task. Seems to be a common issue with appscheduler for differing reasons though. A quick fix for this is to limit the number of job instances to 1. There should be no reason why we would run multiple decoding_cmd's or multiple demodulation_cmd's at the same time. In testing, I have not reproduced the problem after dropping this setting to 1.