Switch record_payloads from using &'b [u8] to bytes::Buf

[imotov]

As we discussed in quickwit-oss/quickwit#2834, we need to store more than a record's content in payload. In case we want to store a command like this:

enum Command<'a> {
     Ingest {
         json_payload: &'a [u8]
     },
     Commit,
     // ... more to come?
}

we will need to write a command type followed by json_payload. Having &[u8] in the interface doesn't allow us to write two chunks of data efficiently. However, if we switch interface to use bytes::Buf it will allows us to continue using &[u8] thanks to impl Buf for &[u8] and at the same time be able to provide access to command code and payload without copying thanks to Buf's support for chaining.

[fulmicoton]

That sounds like the perfect solution.