Refactor storage dir

I propose that we make `StorageDir` the entry point and drop `BlockHandler`.  The core reasons for this are that:

`BlockHandler` is the same thing as java's `Channel`.  Easier to use one std convention than create a new one.
`StorageDir` is the main entry point anyways, since the path is relative to the tier, so `BlockHandler` can't be the gateway since it's path depends on the `StorageDir`.
That would need to be different code since they act very different.
On Oct 20, 2014 12:12 AM, "shimingfei" <notifications@github.com> wrote:

> In core/src/main/java/tachyon/worker/hierarchy/StorageDir.java:
>
> > @@ -571,4 +566,14 @@ public boolean unlockBlock(long blockId, long userId) {
> >      mLockedBlocksPerUser.remove(userId, blockId);
> >      return true;
> >    }
> > +
> > +  public boolean addBlock(long blockId) {
> > +    File path = new File(getBlockFilePath(blockId));
>
> currently, only local SSD and HDD is supported, but we may support more
> storage later, such as HDFS
>
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> <https://github.com/amplab/tachyon/pull/461/files#r19070422>.
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