Cache populations for stop in weighted avg svc freq

By popular demand (Issue #392) caching for db lookups on demographics.
This is more thread safe (not sure if it will compile):

```scala
import scala.collection.JavaConversions._

val popMap: mutable.ConcurrentMap = new java.util.concurrent.ConcurrentHashMap
```

Then use the concurrent map functions: 
http://www.scala-lang.org/api/2.10.3/index.html#scala.collection.mutable.ConcurrentMap