Changes to WQ for the experimental work in TCC journal submission

The changes introduce:
1. reporting totals of local cores, disk, gpu and memory as min and max values. These are now reported in WQ stats but are currently undocumented. We might want to revisit this as part of #276. 
2. an experimental feature that blocks the master from dispatching tasks until a specified number of workers connect. This was used in running experiments for the journal.
3. new fields in WQ task struct that record the time of (stdout) result retrieval since we fetch results much earlier than output files.
4. documented API to set and get the current master bandwidth. The set API gives same behavior as the "--bandwidth" option in wq_worker for limiting foreman bandwidth.