Missing 'GETINFO md/all'

Hi Nick. The control spec has GETINFO options to fetch a microdescriptor via its fingerprint or nickname ('md/id/' and 'md/name/') but no method for getting them all (like 'ns/all' or 'desc/all-recent').

Processing all descriptors is a pretty common need, hence the priority (feel free to lower it if appropriate). In the meantime I'll hack around this in stem by having controller read microdescriptors from the data directory.