Tons left to do and lots of churn:
1. No state saving
2. No shutdown or gc
3. Removed AR factory *for now*
4. Made all "Config" structs local to the package they configure
5. Added allocID to GC to avoid a lookup
Really hating how many things use *structs.Allocation. It's not bad
without state saving, but if AllocRunner starts updating its copy things
get racy fast.
WARNing when someone has over 50 non-terminal allocs was just too
confusing.
Tested manually with `gc_max_allocs = 10` and bumping a job from `count
= 19` to `count = 21`:
```
2017/11/02 17:54:21.076132 [INFO] client.gc: garbage collection due to number of allocations (19) is over the limit (10) skipped because no terminal allocations
...
2017/11/02 17:54:48.634529 [WARN] client.gc: garbage collection due to number of allocations (21) is over the limit (10) skipped because no terminal allocations
```
The Client.allocs map now contains all AllocRunners again, not just
un-GC'd AllocRunners. Client.allocs is only pruned when the server GCs
allocs.
Also stops logging "marked for GC" twice.
This PR introduces a parallelism limit during garbage collection. This
is used to avoid large resource usage spikes if garbage collecting many
allocations at once.
