Restore compatibility of Log4j API

This restores backward compatibility of `log4j-api` from a user's
perspective and adds comments to classes that should not have breaking
changes.

I have checked all the types in `org.apache.logging.log4j` and those
reachable through inheritance or signature of public methods in other
packages:

```
org.apache.logging.log4j.message.EntryMessage
org.apache.logging.log4j.message.ExitMessage
org.apache.logging.log4j.message.FlowMessage
org.apache.logging.log4j.message.FlowMessageFactory
org.apache.logging.log4j.message.Message
org.apache.logging.log4j.message.MessageFactory
org.apache.logging.log4j.spi.LoggerContext
org.apache.logging.log4j.spi.ExtendedLogger
org.apache.logging.log4j.spi.LoggerContextFactory
org.apache.logging.log4j.spi.LoggerRegistry
org.apache.logging.log4j.spi.StandardLevel
org.apache.logging.log4j.spi.ReadOnlyThreadContextMap
org.apache.logging.log4j.util.BiConsumer
org.apache.logging.log4j.util.ReadOnlyStringMap
org.apache.logging.log4j.util.StringMap
org.apache.logging.log4j.util.StringBuilderFormattable
org.apache.logging.log4j.util.Supplier
org.apache.logging.log4j.util.TriConsumer
```

The only breaking changes I found are `Logger#entry/exit` and
in `MarkerFactory`. I believe we should restore them and have a 100% binary
compatibility with `2.x`, rather then breaking BC for almost nothing.

Author: ppkarwasz

log4j-api/pom.xml

@@ -29,6 +29,7 @@
   <description>The Apache Log4j API</description>
   <properties>
     <maven.javadoc.skip>false</maven.javadoc.skip>
+    <bnd.baseline.skip>false</bnd.baseline.skip>
 
     <!--
       ~ OSGi and JPMS options

log4j-api/src/main/java/org/apache/logging/log4j/Level.java

@@ -18,6 +18,7 @@
 
 import static org.apache.logging.log4j.util.Strings.toRootUpperCase;
 
+import aQute.bnd.annotation.baseline.BaselineIgnore;
 import java.util.Objects;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ConcurrentMap;
@@ -71,6 +72,7 @@
  * used in logging configurations.
  * </p>
  */
+@BaselineIgnore("3.0.0")
 public final class Level implements Comparable<Level> {
 
     private static final Level[] EMPTY_ARRAY = {};

log4j-api/src/main/java/org/apache/logging/log4j/LogManager.java

@@ -40,7 +40,13 @@
 public class LogManager {
 
     /**
-     * Constant holding the name of the root logger (the empty string).
+     * Kept for binary compatibility.
+     */
+    @Deprecated(forRemoval = true)
+    public static final String FACTORY_PROPERTY_NAME = "log4j2.loggerContextFactory";
+
+    /**
+     * The name of the root Logger is {@value #ROOT_LOGGER_NAME}.
      */
     public static final String ROOT_LOGGER_NAME = Strings.EMPTY;