Implement SQLiteDatabase::Rewrite

Rewrite uses the VACUUM command which does exactly what we want. A
specific advertised use case is to compact a database and ensure that
any deleted data is actually deleted.

src/wallet/sqlite.cpp

@@ -181,7 +181,9 @@ void SQLiteDatabase::Open()
 
 bool SQLiteDatabase::Rewrite(const char* skip)
 {
-    return false;
+    // Rewrite the database using the VACUUM command: https://sqlite.org/lang_vacuum.html
+    int ret = sqlite3_exec(m_db, "VACUUM", nullptr, nullptr, nullptr);
+    return ret == SQLITE_OK;
 }
 
 bool SQLiteDatabase::Backup(const std::string& dest) const