cmd/syncthing: Add selectable sha256 package (fixes #3613, fixes #3614)

This adds autodetection of the fastest hashing library on startup, thus
handling the performance regression. It also adds an environment
variable to control the selection, STHASHING=standard (Go standard
library version, avoids SIGILL crash when the minio library has bugs on
odd CPUs), STHASHING=minio (to force using the minio version) or unset
for the default autodetection.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3617

lib/protocol/bep_extensions.go

@@ -12,9 +12,8 @@ import (
 	"fmt"
 	"time"
 
-	"github.com/minio/sha256-simd"
-
 	"github.com/syncthing/syncthing/lib/rand"
+	"github.com/syncthing/syncthing/lib/sha256"
 )
 
 const (

lib/protocol/deviceid.go

@@ -11,7 +11,7 @@ import (
 	"regexp"
 	"strings"
 
-	"github.com/minio/sha256-simd"
+	"github.com/syncthing/syncthing/lib/sha256"
 
 	"github.com/calmh/luhn"
 )