light: primary is more tolerant against truncated logfile
light: use replay_tolerance only after failed replay attempt
light: add syncpos symlink

This is needed for detection of the real end of inconsistencies
after sync as finished. Consistency is only (re-)reached after
a certain amount of logfile data has been sucessfully applied.

This patch remembers the replaylink from the primary at the time
when the sync has finished.

When at least that amount of logfile data has been applied, we
are certain that now we are consistent.