clean up uri-vs-cap terminology, emphasize cap instances instead of URI strings
* "cap" means a python instance which encapsulates a filecap/dircap (uri.py)
* "uri" means a string with a "URI:" prefix
* FileNode instances are created with (and retain) a cap instance, and
generate uri strings on demand
* .get_cap/get_readcap/get_verifycap/get_repaircap return cap instances
* .get_uri/get_readonly_uri return uri strings
* add filenode.download_to_filename() for control.py, should find a better way
* use MutableFileNode.init_from_cap, not .init_from_uri
* directory URI instances: use get_filenode_cap, not get_filenode_uri
* update/cleanup bench_dirnode.py to match, add Makefile target to run it
16 files changed: