Reduce the memory requirement of heap dump compression

Motivation:

The current heap dump compression preset (9) requires way too much
memory (768 MiB at maximum for dictionary), resulting in OOME in many
cases.

Modifications:

- Use the default preset (6) which uses 8 MiB dictionary.
- Do not fail abruptly even when OOME has been raised.

Result:

More stable heap dump acquisition

testsuite/src/main/java/io/netty/testsuite/util/TestUtils.java

@@ -239,7 +239,7 @@ public final class TestUtils {
         });
 
         final byte[] buf = new byte[65536];
-        final LZMA2Options options = new LZMA2Options(9);
+        final LZMA2Options options = new LZMA2Options(LZMA2Options.PRESET_DEFAULT);
 
         for (File file: files) {
             final String filename = file.toString();
@@ -277,8 +277,8 @@ public final class TestUtils {
                 }
                 out.close();
                 in.close();
-            } catch (Exception e) {
+            } catch (Throwable t) {
-                logger.warn("Failed to compress the heap dump: {}", xzFilename, e);
+                logger.warn("Failed to compress the heap dump: {}", xzFilename, t);
             } finally {
                 if (in != null) {
                     try {