MINOR: Fix race condition on shutdown of verifiable producer
MINOR: Fix race condition on shutdown of verifiable producer

We've seen `ReplicaVerificationToolTest.test_replica_lags` fail occasionally due to errors such as the following:
```
RemoteCommandError: ubuntuworker7: Command 'kill -15 2896' returned non-zero exit status 1. Remote error message: bash: line 0: kill: (2896) - No such process
```
The problem seems to be a shutdown race condition when using `max_messages` with the producer. The process may already be gone which will cause the signal to fail.

Author: Jason Gustafson <jason@confluent.io>

Reviewers: Gwen Shapira

Closes #6906 from hachikuji/fix-failing-replicat-verification-test

noslowerdna/kafka,guozhangwang/kafka,KevinLiLu/kafka,TiVo/kafka,lindong28/kafka,Chasego/kafka,KevinLiLu/kafka,Chasego/kafka,apache/kafka,TiVo/kafka,noslowerdna/kafka,sslavic/kafka,Chasego/kafka,TiVo/kafka,KevinLiLu/kafka,sslavic/kafka,TiVo/kafka,sslavic/kafka,lindong28/kafka,apache/kafka,noslowerdna/kafka,KevinLiLu/kafka,guozhangwang/kafka,guozhangwang/kafka,Chasego/kafka,lindong28/kafka,apache/kafka,lindong28/kafka,noslowerdna/kafka,apache/kafka,guozhangwang/kafka,sslavic/kafka