all: Support multiple device connections (fixes #141) (#8918)

This adds the ability to have multiple concurrent connections to a single device. This is primarily useful when the network has multiple physical links for aggregated bandwidth. A single connection will never see a higher rate than a single link can give, but multiple connections are load-balanced over multiple links.

It is also incidentally useful for older multi-core CPUs, where bandwidth could be limited by the TLS performance of a single CPU core -- using multiple connections achieves concurrency in the required crypto calculations...

Co-authored-by: Simon Frei <freisim93@gmail.com>
Co-authored-by: tomasz1986 <twilczynski@naver.com>
Co-authored-by: bt90 <btom1990@googlemail.com>

lib/config/config.go

@@ -25,6 +25,7 @@ import (
 	"github.com/syncthing/syncthing/lib/fs"
 	"github.com/syncthing/syncthing/lib/netutil"
 	"github.com/syncthing/syncthing/lib/protocol"
+	"github.com/syncthing/syncthing/lib/sliceutil"
 	"github.com/syncthing/syncthing/lib/structutil"
 )
 
@@ -564,8 +565,7 @@ func ensureNoUntrustedTrustingSharing(f *FolderConfiguration, devices []FolderDe
 		}
 		if devCfg := existingDevices[dev.DeviceID]; devCfg.Untrusted {
 			l.Warnf("Folder %s (%s) is shared in trusted mode with untrusted device %s (%s); unsharing.", f.ID, f.Label, dev.DeviceID.Short(), devCfg.Name)
-			copy(devices[i:], devices[i+1:])
-			devices = devices[:len(devices)-1]
+			devices = sliceutil.RemoveAndZero(devices, i)
 			i--
 		}
 	}
@@ -601,9 +601,7 @@ func filterURLSchemePrefix(addrs []string, prefix string) []string {
 			continue
 		}
 		if strings.HasPrefix(uri.Scheme, prefix) {
-			// Remove this entry
-			copy(addrs[i:], addrs[i+1:])
-			addrs = addrs[:len(addrs)-1]
+			addrs = sliceutil.RemoveAndZero(addrs, i)
 			i--
 		}
 	}