i2psnark: Add checks for dup data dirs (ticket #2291)

Add two HashMaps for faster dup checks

apps/i2psnark/java/src/org/klomp/snark/Snark.java

@@ -1229,6 +1229,8 @@ public class Snark
               baseFile = new File(rootDataDir, base);
           else
               baseFile = new SecureFile(rootDataDir, base);
+          if (baseFile.exists())
+              throw new IOException("Data location already exists: " + baseFile);
           // The following two may throw IOE...
           storage = new Storage(_util, baseFile, metainfo, this, false);
           storage.check();