// Uses the Mutable Resource metadata currently loaded in the resources map entry.
// Note that a Feed update cannot span chunks, and thus has a MAX NET LENGTH 4096, INCLUDING update header data and signature.
// It is the caller's responsibility to make sure that this data is not stale.
// This results in a max payload of `maxUpdateDataLength` (check update.go for more details)
// Note that a Mutable Resource update cannot span chunks, and thus has a MAX NET LENGTH 4096, INCLUDING update header data and signature. An error will be returned if the total length of the chunk payload will exceed this limit.
// An error will be returned if the total length of the chunk payload will exceed this limit.
// Update can only check if the caller is trying to overwrite the very last known version, otherwise it just puts the update
// Update can only check if the caller is trying to overwrite the very last known version, otherwise it just puts the update