No buffer allocation on the hot path about lightyear HOT 1 OPEN

Places where allocations can be re-used:

1. when we receive &[u8] from the network, we need to copy them into a Buffer used by bitcode to deserialize the bytes into a Packet. We can keep re-using the same buffer instead of allocating a new one every time. (if we read from io in parallel, we can use a pool of buffers)
2. in the message manager (one per connection), we need a buffer to deserialize the SingleData bytes into the actual message.
3. in the message manager (one per connection), we need a buffer to serialize the byte into a SingleData.
4. In FragmentReceiver, we allocate a vec to hold the bytes until we have received the entire packet.

For 1), 2), 3); we reuse buffers.
TODO: we need to use a buffer pool for 4)
Are there others?

from lightyear.