recv_recover_page(): Correct a debug assertion to refer to recv_sys.lsn,
which may be ahead of log_sys.lsn during non-final recovery batches.
In commit 685d958e (MDEV-14425)
when some redundant LSN fields were removed,
log_sys.log.scanned_lsn had been replaced with a reference to
log_sys.lsn instead of the more appropriate recv_sys.lsn.

recv_scan_log(): Remove a redundant call to log_sys.set_recovered_lsn().
It suffices to adjust the log_sys.lsn after parsing (and before starting
to apply) records for the last batch.

Note: Normally, log_sys.lsn must be the latest log sequence number.
Before the final recovery batch, this may be safely violated, because
log_write_up_to() will be a no-op. That function will be invoked by the
buf_flush_page_cleaner thread to initiate writes of recovered pages.