Adding a resolved timestamp to phosphor-logging's event logs

[Matt Spinler]

To those that use phosphor-logging's openbmc event logs:

I just put up a phosphor-dbus-interfaces change 
https://gerrit.openbmc-project.xyz/c/openbmc/phosphor-dbus-interfaces/+/29734
to add a new ResolvedTimestamp property to the OpenBMC event log entry.

Next I'll update phosphor-logging to set that timestamp when the Resolved
property changes, so that users can know when an error was resolved.

Review if interested.

Thanks