sync CatMap to avoid concurrent map access.

[glycerine]

Add sync.Lock() and sync.Unlock() around reads and writes to the CatMap. Also add full copying of the CatMap instead of sharing. That may reduce contention.

Fixes #20

[glycerine]

I originally had it as Mut, but then it got confusing where the mutex was coming from -- because the CatMap is embedded in other structures the origin of the mutex was getting lost when calling in through those inheriting structs.

in densecatfeature.go:74 in particular...

https://github.com/lytics/CloudForest/pull/23/files#diff-e6bce0ff1df02a9f3541e95ad47641aeR74

[timkaye11]

good point @glycerine