For each supported adapter clock and for the server system clock,
save the current frequency correction value every 60 seconds. If the server is rebooted
sfptpd is restarted, the last saved value is used to continue
clock frequency adjustment rather than revert to a zero value which would delay
re-synchronization to an external master clock (or re-synchronization to the LRC if the
adapter clock is not the active clock receiving from the remote master clock).
Frequency correction files are saved in /var/lib/sfptpd. Refer to Table 1 for details.