Path filtering on "dump cache" asok
Sometimes we will want to get a detailed dump of the cache for one particular subtree, so implement a mode that will traverse down whatever is in cache beneath a particular path. Perhaps also allow a `depth` parameter so that one could e.g. list a directory without all its descendents too.
Like "ceph daemon mds.a dump cache", but with an extra argument that limits the output to a particular sub-path. Code is in MDCache::dump_path, entered through MDSRank::handle_asok_command. Confusingly the existing asok command already has a 'path' argument, but that's the output filename, not a filter.