Investigate dynamic payload lengths

A few possibilities:

If most variable length payloads are < 255 then could store as a single byte
Is it possible to have payloads > 2^16 bytes? Should at least assert if that is the case because it would break the compression format which assumes 2 bytes.

Payload offsets could be stored once every 16 records in memory and then add the sizes. Would reduce the in-memory overhead from 64 to 4 bytes per 16 records.