Migrate HDFSDirectory from solr to lucene-hadoop [LUCENE-6536]

I am currently working on a search engine that is throughput orientated and works entirely in apache-spark.

As part of this, I need a directory implementation that can operate on HDFS directly. This got me thinking, can I take the one that was worked on so hard for solr hadoop.

As such I migrated the HDFS and blockcache directories out to a lucene-hadoop module.

Having done this work, I am not sure if it is actually a good change, it feels a bit messy, and I dont like how the Metrics class gets extended and abused.

Thoughts anyone

---
Migrated from [LUCENE-6536](https://issues.apache.org/jira/browse/LUCENE-6536) by Greg Bowyer (@GregBowyer)
Attachments: [LUCENE-6536.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-6536/LUCENE-6536.patch)

Review the attached LUCENE-6536.patch and the migrated HDFS and blockcache directories described in the issue. Examine how the lucene-hadoop module uses the Metrics class, then determine whether the migration is appropriate and what acceptance criteria are needed; the issue does not define a completed outcome.

Written by the indexing model from the issue text.