walletdb: Introduce AddRef and RemoveRef functions

Refactor mapFileUseCount increment and decrement to separate functions
AddRef and RemoveRef

src/wallet/bdb.h

@@ -121,6 +121,11 @@ public:
      */
     bool Rewrite(const char* pszSkip=nullptr);
 
+    /** Indicate the a new database user has began using the database. */
+    void AddRef();
+    /** Indicate that database user has stopped using the database and that it could be flushed or closed. */
+    void RemoveRef();
+
     /** Back up the entire database to a file.
      */
     bool Backup(const std::string& strDest) const;
@@ -212,6 +217,7 @@ protected:
     bool fReadOnly;
     bool fFlushOnClose;
     BerkeleyEnvironment *env;
+    BerkeleyDatabase& m_database;
 
 public:
     explicit BerkeleyBatch(BerkeleyDatabase& database, const char* pszMode = "r+", bool fFlushOnCloseIn=true);