send logs to elasticsearch in bulk

This MR changes the way of sending logs from the build to elasticsearch. Instead of using index method at each log line available, it uses bulk method sending aggregated logs from last 5 seconds. The reason is to limit the rate of queries (I was getting quite frequently emails titled "High number of DNS queries", and saying that the host is sending high rate of queries for the last hour (e.g. ~100 requests/sec)).

To be merged after !16 (merged) !18 (merged) !20 (merged) !21 (merged)