Enforce payload size limit on sent message

Following up discussion on #112 .

"To actually enforce the limit, we could do something like this maybe:
``src/network/wire/lib.zig``
```
const MAX_PAYLOAD_SIZE: u32 = 32 * 1024 * 1024

const payload_len: u32 = @intCast(payload.len);
if (payload_len > MAX_PAYLOAD_SIZE) {
    return someerror
}
```
"