Move to crypto/rand for nonce generation (#3324)

Signed-off-by: Matthias Hanel <mh@synadia.com>

server/nkey.go

@@ -14,6 +14,7 @@
 package server
 
 import (
+	"crypto/rand"
 	"encoding/base64"
 )
 
@@ -41,6 +42,6 @@ func (s *Server) nonceRequired() bool {
 func (s *Server) generateNonce(n []byte) {
 	var raw [nonceRawLen]byte
 	data := raw[:]
-	s.prand.Read(data)
+	rand.Read(data)
 	base64.RawURLEncoding.Encode(n, data)
 }

server/server.go

@@ -114,7 +114,6 @@ type Server struct {
 	stats
 	mu                  sync.RWMutex
 	kp                  nkeys.KeyPair
-	prand               *rand.Rand
 	info                Info
 	configFile          string
 	optsMu              sync.RWMutex
@@ -378,7 +377,6 @@ func NewServer(opts *Options) (*Server, error) {
 		kp:                 kp,
 		configFile:         opts.ConfigFile,
 		info:               info,
-		prand:              rand.New(rand.NewSource(time.Now().UnixNano())),
 		opts:               opts,
 		done:               make(chan bool, 1),
 		start:              now,