lib/protocol, lib/discover, lib/db: Use protocol buffer serialization (fixes #3080)

This changes the BEP protocol to use protocol buffer serialization
instead of XDR, and therefore also the database format. The local
discovery protocol is also updated to be protocol buffer format.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3276
LGTM: AudriusButkevicius

lib/protocol/compression.go

@@ -4,13 +4,7 @@ package protocol
 
 import "fmt"
 
-type Compression int
-
 const (
-	CompressMetadata Compression = iota // zero value is the default, default should be "metadata"
-	CompressNever
-	CompressAlways
-
 	compressionThreshold = 128 // don't bother compressing messages smaller than this many bytes
 )
 
@@ -31,14 +25,6 @@ var compressionUnmarshal = map[string]Compression{
 	"always":   CompressAlways,
 }
 
-func (c Compression) String() string {
-	s, ok := compressionMarshal[c]
-	if !ok {
-		return fmt.Sprintf("unknown:%d", c)
-	}
-	return s
-}
-
 func (c Compression) GoString() string {
 	return fmt.Sprintf("%q", c.String())
 }