INTEXT-72: Implement realtime search

* Add `SplunkDataReader#realtimeSearch`
* Add `SplunkRunning` test Rule
* Add Real test-case - `RealTimeSearchIntegrationTests`
* Allow empty string for `init-earliest-time` of `<int-splunk:inbound-channel-adapter>`. For realtime search it is interpreted as
`earliest_time = "rt"` and `latest_time="rt"`. However `sliding window` is still valid option for the realtime search as well.
* Polishing for configs
* Polishing `README.md` and add a note regarding realtime search