Remove reflection from log4j-to-jul and log4j-to-slf4j

By removing the reflective instantiation of `LoggerContextFactory` and
`ThreadContextMap`, we make `log4j-to-jul` and `log4j-to-slf4j` more
GraalVM-friendly.

Author: ppkarwasz

log4j-to-jul/src/main/java/org/apache/logging/log4j/tojul/JULProvider.java

@@ -18,6 +18,7 @@
 
 import aQute.bnd.annotation.Resolution;
 import aQute.bnd.annotation.spi.ServiceProvider;
+import org.apache.logging.log4j.spi.LoggerContextFactory;
 import org.apache.logging.log4j.spi.Provider;
 
 /**
@@ -27,7 +28,20 @@
  */
 @ServiceProvider(value = Provider.class, resolution = Resolution.OPTIONAL)
 public class JULProvider extends Provider {
+    private static final LoggerContextFactory CONTEXT_FACTORY = new JULLoggerContextFactory();
+
     public JULProvider() {
-        super(20, "2.6.0", JULLoggerContextFactory.class, null);
+        super(20, CURRENT_VERSION);
+    }
+
+    @Override
+    public LoggerContextFactory getLoggerContextFactory() {
+        return CONTEXT_FACTORY;
+    }
+
+    @Override
+    public String getThreadContextMap() {
+        // JUL does not provide an MDC implementation
+        return NO_OP_CONTEXT_MAP;
     }
 }

log4j-to-slf4j/src/main/java/org/apache/logging/slf4j/SLF4JProvider.java

@@ -18,14 +18,30 @@
 
 import aQute.bnd.annotation.Resolution;
 import aQute.bnd.annotation.spi.ServiceProvider;
+import org.apache.logging.log4j.spi.LoggerContextFactory;
 import org.apache.logging.log4j.spi.Provider;
+import org.apache.logging.log4j.spi.ThreadContextMap;
 
 /**
  * Bind the Log4j API to SLF4J.
  */
 @ServiceProvider(value = Provider.class, resolution = Resolution.OPTIONAL)
 public class SLF4JProvider extends Provider {
+
+    private static final LoggerContextFactory CONTEXT_FACTORY = new SLF4JLoggerContextFactory();
+    private static final ThreadContextMap THREAD_CONTEXT_MAP = new MDCContextMap();
+
     public SLF4JProvider() {
-        super(15, "2.6.0", SLF4JLoggerContextFactory.class, MDCContextMap.class);
+        super(15, CURRENT_VERSION);
+    }
+
+    @Override
+    public LoggerContextFactory getLoggerContextFactory() {
+        return CONTEXT_FACTORY;
+    }
+
+    @Override
+    public ThreadContextMap getThreadContextMapInstance() {
+        return THREAD_CONTEXT_MAP;
     }
 }