Abstract MetadataStore in S3Guard into a common module.

Open this JIRA to discuss if we should move MetadataStore in S3Guard into a common module.

Based on this work, other filesystem or object store can implement their own metastore for optimization (known issues like consistency problem and metadata operation performance). stevel@apache.org and other guys have done many base and great works in S3Guard. It is very helpful to start work. I did some perf test in HADOOP-14098, and started related work for Aliyun OSS. Indeed there are still works to do for S3Guard, like metadata cache inconsistent with S3 and so on. It also will be a problem for other object store. However, we can do these works in parallel.

stevel@apache.org fabbri drankye Any suggestion is appreciated.