This allows batch replication to basically do not
attempt to copy objects that do not have read quorum.
This PR also allows walk() to provide custom
values for quorum under batch replication, and
key rotation.
* Reduce allocations
* Add stringsHasPrefixFold which can compare string prefixes, while ignoring case and not allocating.
* Reuse all msgp.Readers
* Reuse metadata buffers when not reading data.
* Make type safe. Make buffer 4K instead of 8.
* Unslice
