DCP 'commit' consumer/producer discrepancy of 1 byte leads to DCP halt

In 6.5 we added namespace support in KV-engine, a technique used by durability and a stepping stone towards collections.

This means that a key in many places (disk/memory) gets a namespace prefix, all committed keys get the "default" collection prefix of a single empty byte (0) and prepared writes get a a prefix of 2. This means that when the user stores a document with a 3-byte key of "JIM", in memory it gets 4 bytes "0JIM".

When DCP replicates that document, there is no namespace support over the wire and "0JIM" gets replicated as "JIM".

This leads to an issue in buffer-ack when we do DCP commit.

The producer is recording the namepaced length but the consumer is acking for the un-namespaced size - a 1 byte difference.