Optimize readDir where filtering is required (#5810)

* Switch to Java NIO for readDirInternal

* Revert "Switch to Java NIO for readDirInternal"

This reverts commit ab7601f.

* Tactical fix

Author: Mulgish

vertx-core/src/main/java/io/vertx/core/file/impl/FileSystemImpl.java

@@ -842,9 +842,10 @@ public List<String> perform() {
           } else {
             FilenameFilter fnFilter;
             if (filter != null) {
+              Pattern fnPattern = Pattern.compile(filter);
               fnFilter = new FilenameFilter() {
                 public boolean accept(File dir, String name) {
-                  return Pattern.matches(filter, name);
+                  return fnPattern.matcher(name).matches();
                 }
               };
             } else {