Avoid UUID.randomUUID() in startup code

This is done because bootstrapping the plumbing
needed by the JDK to produce a UUID value
is expensive, it thus doesn't make sense to
pay this cost when the property isn't actually
needed

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

@@ -45,7 +45,7 @@ static File setupCacheDir(String fileCacheDir) {
 
     // the cacheDir will be suffixed a unique id to avoid eavesdropping from other processes/users
     // also this ensures that if process A deletes cacheDir, it won't affect process B
-    String cacheDirName = fileCacheDir + "-" + UUID.randomUUID();
+    String cacheDirName = fileCacheDir + "-" + System.nanoTime();
     File cacheDir = new File(cacheDirName);
     // Create the cache directory
     try {

vertx-core/src/main/java/io/vertx/core/impl/deployment/DefaultDeploymentManager.java

@@ -36,7 +36,7 @@ public DefaultDeploymentManager(VertxImpl vertx) {
   }
 
   private String generateDeploymentID() {
-    return UUID.randomUUID().toString();
+    return Long.valueOf(System.nanoTime()).toString();
   }
 
   public Future<Void> undeploy(String deploymentID) {