use uuids in a more compact form

Minor minor optimization. We're currently using uuids in hex format. It would be more efficient to use something like base64. hex has a fanout of 16, base64 has 64 obviously. Would make S3 access more efficient (b/c of quicker fanout), & would shorten the strings we use & store internally a little.

Base64 is just an example, it could be any encoding that represents more than 16 values / char, and which s3 would support.