[TACHYON-220] WIP Comments describing issues and possible solutions 

Some initial comments to help improve the performance of master info. From what I gather there is locking on stack local objects that are never really put back into the map. I would appreciate suggestions before implementing it, hence the WIP tag.
Agree, lets stop talking terms and lets go to code since that is not something we can disagree on.

```scala
scala> import java.util.concurrent._
import java.util.concurrent._

scala> class Foo(var name: String)
defined class Foo

scala> val data = new ConcurrentHashMap[String, Foo]()
data: java.util.concurrent.ConcurrentHashMap[String,Foo] = {}

scala> data.put("foo", new Foo("foo"))
res0: Foo = null

scala> data.get("foo")
res1: Foo = Foo@52380405

scala> res1.name = "bob"
res1.name: String = bob

scala> data.get("foo").name
res2: String = bob
```

As we see here, `foo`'s state will be mutated. This mutation is currently shared by multiple threads and so is unsafe without locking.  As stated below, if we want to make these mutations stack local until we add it to the map, then the class `Foo` must be copied.