Add CBOR-friendly bulk format

[clintongormley]

The current bulk format in JSON uses the \n as end-of-line markers, allowing the coordinating node to parse just the metadata line, then skip over the body line.

CBOR doesn't have any disallowed characters or character sequences, so this mechanism will not work. Instead we should have a CBOR specific format which looks like this:

{ metadata line }
int_length_of_body_line
{ body line }

The int_length_of_body_line would allow us to skip to the next metadata line.

For the delete action, no int_length_of_body_line would be required, as we already know that it can't have a body.

Depends on #7640

[tarass]

Any updates?

CBOR would be great compliment to improved number handling in 5.0 release considering it has variable size numerics.

[tlrx]

Since CBOR allows embedded CBOR data item, maybe we could have a CBOR specific bulk format that is basically an array of data items but use https://tools.ietf.org/html/rfc7049#section-2.4.4.1 for the body item?

[clintongormley]

@tlrx that's an interesting idea... not sure how easy it'd be to fit into our bulk parsing code. Please could you investigate when you have some time?

[vadimkibana]

Two more options how bulk requests could be achieved:

1. CBOR documents can simply follow one after another, until the end of HTTP request is reached—this way CBOR parser can simply parse them one by one sequentially.
2. CBOR specification has an Indefinite length array type, which is an unknown size array, which is terminated by a break 0xFF octet. This way the _bulk endpoint payload could be an indefinite length CBOR array, where each array element is a separate request payload.