balancer: maintain a single instance of SGEStats

This prevents resetting the internal SGEStats job data on each iteration
which was causing the average duration and wait time stats to alternate
from a positive value to 0.

See:

https://github.com/jtriley/StarCluster/commit/1e9cbed49888b0b6c9f9582fb28e42d92e64875d#commitcomment-3575015