Unify DeleteFile and DeleteWalFiles

Summary:
This is to simplify rocksdb public APIs and improve the code quality.
Created an additional parameter to ParseFileName for log sub type and improved the code for deleting a wal file.
Wrote exhaustive unit-tests in delete_file_test
Unification of other redundant APIs can be taken up in a separate diff

Test Plan: Expanded delete_file test

Reviewers: dhruba, haobo, kailiu, sdong

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D13647

include/utilities/stackable_db.h

@@ -144,8 +144,8 @@ class StackableDB : public DB {
     return sdb_->GetSortedWalFiles(files);
   }
 
-  virtual Status DeleteWalFiles(const VectorLogPtr& files) override {
-    return sdb_->DeleteWalFiles(files);
+  virtual Status DeleteFile(std::string name) override {
+    return sdb_->DeleteFile(name);
   }
 
   virtual Status GetUpdatesSince(SequenceNumber seq_number,