Option to send only aggregate CPU stats

This introduces an option `SHH_CPU_AGGR` which prevents shh from sending stats for each core (thereby only sending the aggregate `cpu` line).

Use case: we have a server that reports 32 cores in `/proc/stat` and this poller very rapidly becomes a disk space hog. I could address this by changing the retention on `cpu(\d+)` fields, but I'd rather not send the stats at all.