The function log_header_read() was only used during server startup,
and it will mostly be used only for reading checkpoint information
from pre-MDEV-14425 format redo log files.

Let us replace the function with more direct calls, so that
it is clearer what is going on. It is not strictly necessary to
hold any mutex during this operation, and because there will be
only a limited number of operations during early server startup,
it is not necessary to increment any I/O counters.