Apparent root cause of {T308943} and {T309371}
```
Max connection count for user jenkins-bot exceeded, rejecting new connection. currentSessionCount = 4, maxSessionCount = 4
```
Gerrit itself itself shows 3 open connections:
```
$ ssh -p 29418 gerrit.wikimedia.org gerrit show-connections | grep jenkins
716fc32f jenkins-bot contint2001.wikimedi
944d965d jenkins-bot contint2001.wikimedi
f45092f9 jenkins-bot contint2001.wikimedi
```
@hashar took a stack dump (`kill -SIGUSR2 <pidof zuul>`)
{P31015}
The three connections:
```
lsof -p 15751|grep gerrit
zuul-serv 15751 zuul 8u IPv6 295859537 0t0 TCP contint2001.wikimedia.org:57134->gerrit.wikimedia.org:29418 (ESTABLISHED)
zuul-serv 15751 zuul 17u IPv6 173274822 0t0 TCP contint2001.wikimedia.org:39456->gerrit.wikimedia.org:29418 (ESTABLISHED)
zuul-serv 15751 zuul 24u IPv6 295859542 0t0 TCP contint2001.wikimedia.org:57136->gerrit.wikimedia.org:29418 (ESTABLISHED)
```
We should probably figure out //why// it's happening, and maybe see if setting up an icinga alert is possible?