use base62 encoding for storage indexes, on disk and in verifier caps, and in logging...

use base62 encoding for storage indexes, on disk and in verifier caps, and in logging and diagnostic tools
base62 encoding fits more information into alphanumeric chars while avoiding the troublesome non-alphanumeric chars of base64 encoding.  In particular, this allows us to work around the ext3 "32,000 entries in a directory" limit while retaining the convenient property that the intermediate directory names are leading prefixes of the storage index file names.