Comment 1 for bug 1171699

A preliminary fix that does not hold the mutex during the log read fixes a performance regression of some 30% on a Sysbench write-only workload and makes its performance ~match the case of log tracking disabled.

This fix still acquires the mutex for the log_group_calc_lsn_offset() call, which does not seem necessary neither, but removing that seems to be 1) more involved; 2) not that urgent now.