Expose IEventLogger to make event logging pluggable

For the first time, "Event Logger" feature is designed to make implementation pluggable, so that's why IEventLogger exists, but we didn't have actual use case other than just writing them to the file at that time, so we just simplified the case.

Now we have use case which also write events to file, but with awareness of structure of event so that it can be easily parseable from log feeder. We would want to have custom IEventLogger to represent event as our own format in this case.

There's another issue as well: EventInfo has `ts` which stores epoch but it's defined as String, not long.