This updates our key generation to use Ed25519 keys/certificates for
sync connections. Certificates for browser use remain ECDSA for wider
compatibility.
Ed25519 is more modern and has fewer concerns for the future than the
ECDSA curves we used previously. It is supported from Go 1.13 and
forwards, which is Syncthing 1.3.0 (October 2019).
### Purpose
Treat X-Forwarded-For as a comma-separated string to prevent nil IP being returned by the Discovery Server
### Testing
Unit Tests implemented
Testing with a Discovery Client can be done as follows:
```
A simple example to replicate this entails running Discovery with HTTP, use Nginx as a reverse proxy and hardcode (as an example) a list of IPs in the X-Forwarded-For header.
1. Send an Announcement with tcp://0.0.0.0:<some-port>
2. Query the DeviceID
3. Observe the returned IP Address is no longer nil; i.e. `tcp://<nil>:<some-port>`
```
Jakob Borg
vapatel2
Audrius Butkevicius