Introduce simple refresh strategy

scm-webapp/src/main/java/sonia/scm/security/PercentageJwtAccessTokenRefreshStrategy.java

@@ -0,0 +1,25 @@
+package sonia.scm.security;
+
+import java.time.Clock;
+
+public class PercentageJwtAccessTokenRefreshStrategy implements JwtAccessTokenRefreshStrategy {
+
+  private final Clock clock;
+  private final float refreshPercentage;
+
+  public PercentageJwtAccessTokenRefreshStrategy(float refreshPercentage) {
+    this(Clock.systemDefaultZone(), refreshPercentage);
+  }
+
+  PercentageJwtAccessTokenRefreshStrategy(Clock clock, float refreshPercentage) {
+    this.clock = clock;
+    this.refreshPercentage = refreshPercentage;
+  }
+
+  @Override
+  public boolean shouldBeRefreshed(JwtAccessToken oldToken) {
+    long liveSpan = oldToken.getExpiration().getTime() - oldToken.getIssuedAt().getTime();
+    long age = clock.instant().toEpochMilli() - oldToken.getIssuedAt().getTime();
+    return age/liveSpan > refreshPercentage;
+  }
+}