FuelP2P RequestResponse - Check conversions from usize to u64

[acerone85]

Context

We currently define

pub struct PostcardCodec {
    /// Used for `max_size` parameter when reading Response Message
    /// Necessary in order to avoid DoS attacks
    /// Currently the size mostly depends on the max size of the Block
    max_response_size: usize,
}

but we always convert max_response_size to a u64 when reading it. We should define max_response_size to be a u64 directly.