Refactor: Use std::string not std::string_view in copy_start()

Because std::string guarantees that there is a \0 at the end which we
need for the C interface of the PostgreSQL library.

Gets rid of clang-tidy warnings.

Author: joto

src/db-copy.cpp

@@ -221,7 +221,7 @@ void db_copy_thread_t::thread_t::start_copy(
     }
 
     sql.push_back('\0');
-    m_db_connection.copy_start(sql.data());
+    m_db_connection.copy_start(to_string(sql));
 
     m_inflight = target;
 }

src/pgsql.cpp

@@ -136,12 +136,12 @@ pg_result_t pg_conn_t::exec(std::string const &sql) const
     return exec(sql.c_str());
 }
 
-void pg_conn_t::copy_start(std::string_view sql) const
+void pg_conn_t::copy_start(std::string const &sql) const
 {
     assert(m_conn);
 
     log_sql("(C{}) {}", m_connection_id, sql);
-    pg_result_t const res{PQexec(m_conn.get(), sql.data())};
+    pg_result_t const res{PQexec(m_conn.get(), sql.c_str())};
     if (res.status() != PGRES_COPY_IN) {
         throw fmt_error("Database error on COPY: {}", error_msg());
     }

src/pgsql.hpp

@@ -241,7 +241,7 @@ class pg_conn_t
      */
     void set_config(char const *setting, char const *value) const;
 
-    void copy_start(std::string_view sql) const;
+    void copy_start(std::string const &sql) const;
     void copy_send(std::string_view data, std::string_view context) const;
     void copy_end(std::string_view context) const;