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.

PR-URL: #28737
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Rich Trott <rtrott@gmail.com>

src/env.h

@@ -609,6 +609,13 @@ 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,