Improve thread-safe initialization of SLF4J

http://jira.qos.ch/browse/SLF4J-167 has been fixed in SLF4J 1.7.14
so that the initialization trick can be removed from SonarQube.

pom.xml

@@ -63,7 +63,7 @@
     <h2.version>1.3.176</h2.version>
     <jetty.version>8.1.12.v20130726</jetty.version>
     <logback.version>1.1.3</logback.version>
-    <slf4j.version>1.7.12</slf4j.version>
+    <slf4j.version>1.7.21</slf4j.version>
     <tomcat.version>8.0.32</tomcat.version>
     <elasticsearch.version>2.3.3</elasticsearch.version>
     <orchestrator.version>3.13-build640</orchestrator.version>

server/sonar-process/src/main/java/org/sonar/process/LogbackHelper.java

@@ -48,20 +48,7 @@ public class LogbackHelper {
   public static final String MAX_FILES_PROPERTY = "sonar.log.maxFiles";
 
   public LoggerContext getRootContext() {
-    org.slf4j.Logger logger;
-    while (!((logger = LoggerFactory.getLogger(Logger.ROOT_LOGGER_NAME)) instanceof Logger)) {
-      // It occurs when the initialization step is still not finished because of a race condition
-      // on ILoggerFactory.getILoggerFactory
-      // http://jira.qos.ch/browse/SLF4J-167
-      // Substitute loggers are used.
-      // http://www.slf4j.org/codes.html#substituteLogger
-      try {
-        Thread.sleep(10);
-      } catch (InterruptedException e) {
-        // Ignore
-      }
-    }
-    return ((Logger) logger).getLoggerContext();
+    return ((Logger) LoggerFactory.getLogger(Logger.ROOT_LOGGER_NAME)).getLoggerContext();
   }
 
   public LoggerContextListener enableJulChangePropagation(LoggerContext loggerContext) {