* rust volume: accept odd-length needle id hex in file ids
Go formats the needle id with strconv.FormatUint and parses it back with
strconv.ParseUint, neither of which pads to an even number of hex digits.
hex::decode rejected such file ids with "Odd number of digits", so
volume.fsck could not purge orphans from a rust volume server. Parse the
needle id and cookie with from_str_radix, matching Go's ParseNeedleId
and ParseCookie.
* storage: emit even-length needle id hex in NeedleId.FileId
volume.fsck and volume.check_disk build purge file ids here with unpadded
FormatUint hex, while every other fid formatter strips whole leading zero
bytes. Pad to even length so the output matches the canonical fid format
and strict hex parsers accept it.
Chris Lu