Fix cassandra protocol null handling

[rukai]

Previously our implementation of the cassandra protocol was succesfully decoding null values but when it tried to reencode the null value it would output the bytes 00 00 00 04 FF FF FF FF instead of just FF FF FF FF.
This resulted in e.g. null ip addresses showing up as 255.255.255.255 and integers as -1.

Unfortunately solving this requires completely rewriting cassandra-protocols encoding implementation because of some poor fundamental decisions made there.
The problem is that the recursive generation (for e.g. List and Map types) relies on the Bytes various impl From<T> for Bytes.
However its impossible to represent a Null value on a Bytes, instead the Bytes needs to be wrapped in a CBytes in order to represent Null
This makes it impossible to handle both recursive types (list/map/set etc) and Null values using cassandra-protocol.

In order to get this working immediately for us I've just implemented the changes directly in shotover, its only a little more code shotover side but the custom implementation is:

• faster due to drastically reducing allocations
• easier to understand, all the logic is right in front of us rather than behind multiple layers of abstraction (IDE's have very poor support for navigating into into() implementations)
• we can look to upstream it in the future, but getting it working within shotover is a good first step, it at least illustrates what a good solution should look like when we go to upstream it.

[conorbros]

Is it worth making a PR to cdrs-tokio or do you think just keeping it in Shotover is the best way to go?

I'd be happy to merge as is, but wondering how about future plans.

[rukai]

I absolutely want to investigate if we can upstream to cdrs-tokio as soon as we get the chance to, might be a while before we get that chance though.

[benbromhead]

LGTM - I know it probably feels like a workaround at the moment given the underlying limitations, but happy to land this and work out pushing this upstream as mentioned in the comments above.