Generic Send Offload (GSO) Support

This change implements an initial version of GSO support [1][2][3]
for Linux, which improves the effiency of sending data.

The approach taken in this change is to create a buffer which contains
multiple datagrams in `Connection::poll_transmit`. This was picked
over trying to merge packets in the endpoint task, since packets
seem to need to be padded to a common segment size in order to
make them sendable via GSO.

In order to to this in an efficient fashion, the `poll_transmit`
method was restructred. Instead of selecting spaces upfront, it
will now loop through all possible packet spaces, check if there is
pending data to send and create packets and datagrams out of this.

The last packet which was written to a datagram buffer is not
finalized until it is clear whether follow-up packets need to be
written, since we need to know whether this packet should get padded
to MTU length or not.

This change doesn't enable GSO yet, since it will only produce a
single datagram. I will create a separate change for this.

Performance measurements:

**No GSO:**

```
Sent 1073741824 bytes on 1 streams in 4.31s (237.66 MiB/s)
```

**With GSO (up to 8 packets):**

```
Sent 1073741824 bytes on 1 streams in 3.02s (339.18 MiB/s)
```

[1] http://vger.kernel.org/lpc_net2018_talks/willemdebruijn-lpc2018-udpgso-paper-DRAFT-1.pdf
[2] http://vger.kernel.org/lpc_net2018_talks/willemdebruijn-lpc2018-udpgso-presentation-20181104.pdf
[3] https://lwn.net/Articles/752956/