Make parentLoggingMdcContext typing more restrictive (from `Map<?, …

…?>` to `Map<String, String>`) to be compatible with with latest version of slf4j which now defines `MDC.getCopyOfContextMap()` as `Map<String, String>`.

Since the typing is more restrictive, this change should be backwards compatible with older versions of the slf4j api code.

src/main/java/org/subethamail/smtp/server/Session.java

@@ -46,7 +46,7 @@ public class Session implements Runnable, MessageContext
 	 * thread is the one which calls the constructor. MDC is usually inherited
 	 * by new threads, but this mechanism does not work with executors.
 	 */
-	private final Map<?, ?> parentLoggingMdcContext = MDC.getCopyOfContextMap();
+	private final Map<String, String> parentLoggingMdcContext = MDC.getCopyOfContextMap();
 
 	/**
 	 * Uniquely identifies this session within an extended time period, useful