Review directory locking and Blob interactions

Review resolution of local index directory content vs Blob copy.

There has been wrong understanding of following line acquiring a lock on index directory.

 solrCore.getDirectoryFactory().get(indexDirPath, DirectoryFactory.DirContext.DEFAULT, solrCore.getSolrConfig().indexConfig.lockType);

From Yonik:

A couple things about Directory locking.... the locks were only ever to prevent more than one IndexWriter from trying to modify the same index. The IndexWriter grabs a write lock once when it is created and does not release it until it is closed. 

Directories are not locked on acquisition of the Directory from the DirectoryFactory. See the IndexWriter constructor, where the lock is explicitly grabbed.

Review CorePushPull#pullUpdateFromBlob, ServerSideMetadata and other classes as relevant.