Feature requests based on recent usage

[jjhart]

• Short-circuit out of sendEvents if there is nothing to send; sending empty ingestion batches to the Scalyr backend is confusing/misleading

• Support wildcards in the matcher rules config; support a "match all" conceit too so we don’t even need to specify a particular key (let alone static value, as current)

• Add rate-limited logging if the EventMapper only ever returns null, something like this (here):

int nonNullCount = records.stream()
      .map(eventMapper::createEvent)
      .filter(Objects::nonNull)
      .peek(event -> {
        if (eventBuffer.estimatedSerializedBytes() + event.estimatedSerializedBytes() >= batchSendSizeBytes) {
          sendEvents();
        }
        eventBuffer.addEvent(event);
      })
      .count();

if (nonNullCount == 0) {
  log.rateLimitedWarn(..);
  return;
}