recv_t, recv_t::data_t: Define constructors that copy the log records

Ideally, we should remove recv_sys.buf, RECV_DATA_BLOCK_SIZE,
and recv_sys_justify_left_parsing_buf(), and let the
recv_sys.pages point directly to a buffer of parsed redo log
records.

The RECV_PARSING_BUF_SIZE (size of recv_sys.buf) is only 2MiB,
while the minimum innodb_log_buffer_size (size of log_sys.buf) is 16MiB,
and log_sys.buf is unused during redo log apply!