src: add public virtual destructor for KVStore

As KVStore has derived classes, it is essential to
declare a public virtual destructor in the base
KVStore class. Otherwise, deleting derived class
instances using base class pointers would
potentially cause undefined behaviour.

Additionally, since we are implementing a non-default
destructor, the special member functions have also
been implemented in order to abide by the rule of five.

Author: GauthamBanasandra

src/env.h

@@ -602,6 +602,13 @@ class AsyncRequest : public MemoryRetainer {
 
 class KVStore {
  public:
+  KVStore() = default;
+  virtual ~KVStore() = default;
+  KVStore(const KVStore&) = delete;
+  KVStore& operator=(const KVStore&) = delete;
+  KVStore(KVStore&&) = delete;
+  KVStore& operator=(KVStore&&) = delete;
+
   virtual v8::Local<v8::String> Get(v8::Isolate* isolate,
                                     v8::Local<v8::String> key) const = 0;
   virtual void Set(v8::Isolate* isolate,