grouping-by() problems

[czanik]

Using the latest git snapshot from September 20., I ran into multiple problems using grouping-by(). For details check the related discussion at #4639

It seems to work mostly well with UDP or a single TCP connection, but even then:

• some messages seem to be lost (43 instead of 45 found by grouping-by())
• memory leak: some RAM is never freed, even long after timout() is over (went back from 35GB to 15GB after a while, but the rest was never freed (well, not within half an hour)
• some logs are never written, when sending many unique logs with loggen

When sending logs on multiple connections, even at a low message rate:

• some logs are saved multiple times, their number changes on each run