Use Instant objects to measure time across the code to stay consistent

[Laukkala]

Description
There are multiple different ways to measure time in the project, and it would be good to keep measurement of time consistent.
At least in SyslogRecordEventID.java, there is usage of System.currentTimeMillis, while elsewhere Instant is used.

Use case or motivation behind the feature request
Instant seems to be the prevailing method of measuring time, so it should be used across different classes to stay consistent. There might be slight differences in the way Instant and System classes provide timestamps, and it might lead to confusing errors in the future.