v0.20.0 - init-store enhancements and debug job for XFS

• Extends grootfs init-store to take care of creating and mounting a backing filesystem for the store. The optional flag --store-size-bytes controls the size of the filesystem.
• Drops --uid-mapping and --gid-mapping from grootfs create. The UID and GID mappings for the store must now be specified as part of grootfs init-store.
• Makes GrootFS able to optionally report an extensive set of system metrics that are aimed to help us study and improve the way GrootFS uses XFS.
• Improves store validation logic.
• Removes functionality that was lazily initializing the store upon first grootfs create call.
• Introduces the grootfs-debug BOSH job which is designed to store XFS internal logs when it finds processes being in D state (disk wait or uninterruptible sleep) for a long time. This is to help us investigate further issues with stuck D-state processes that we have experienced in production environments.
• Adds migration path for old (pre-v0.20.0) store formats.