Modify handleRecordsWithHeader(final Collection<SinkRecord> records) to use a better categorization function for events with same metadata

[dtregonning]

Currently like records are batched together if they have the same metadata "index, source, sourcetype, host".

The current batching works by grabbing sequential events and starting a new batch if different. Suggestions have been made to use a better way to split out these batches.

Essentially, this is a categorization function. It is simpler to just implements equal and hashCode functions (or other similar idea) on KafkaHeaderUtility class and then build a hashtable to do categorization. Then it will just be like below

void handleRaw(final Collection<SinkRecord> records) {
    if (connectorConfig.headerSupport) {
        // it worth to refactor out a function to handle header case as it is complicate
        handleRecordsWithHeaders(records)
    } else if (...) {
      ...
    } else {
        ...
    }
}

private void handleRecordsWithHeader(final Collection<SinkRecord> records) {
    map<SplunkSinkRecord, List<SplunkSinkRecord>) recordsWithSameHeaders = new ...
    
    for (SinkRecord r: records) {
        SplunkSinkRecord sr = new SplunkSinkRecord(r)
        if (!recordsWithSameHeaders.contains(sr)) {
            recordsWithSameHeaders[sr] = new ArrayList<>()
        }
        recordsWithSameHeaders[sr].add(sr)
    }

    // send categorized events out
}

[dtregonning]

fixed in #168