Split fs.contract.is-blobstore flag into more descriptive flags for use by contract tests.

The fs.contract.is-blobstore flag guards against execution of several contract tests to account for known limitations with blob stores. However, the name is not entirely accurate, because it's still possible that a file system implemented against a blob store could pass those tests, depending on whether or not the implementation matches the semantics of HDFS. This issue proposes to rename the flag or split it into different flags with different definitions for the semantics covered by the current flag.