[MRELEASE-1162] Release plugin should support parallel execution

**[Steven Schlansker](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=stevenschlansker)** opened **[MRELEASE-1162](https://issues.apache.org/jira/browse/MRELEASE-1162?redirect=false)** and commented

We recently enabled parallel builds with `.mvn/maven.config` to improve build performance.

Next time I tried to release, I am greeted with:

```java
[WARNING] *****************************************************************
[WARNING] * Your build is requesting parallel execution, but this         *
[WARNING] * project contains the following plugin(s) that have goals not  *
[WARNING] * marked as thread-safe to support parallel execution.          *
[WARNING] * While this /may/ work fine, please look for plugin updates    *
[WARNING] * and/or request plugins be made thread-safe.                   *
[WARNING] * If reporting an issue, report it against the plugin in        *
[WARNING] * question, not against Apache Maven.                           *
[WARNING] *****************************************************************
[WARNING] The following plugins are not marked as thread-safe in My Build:
[WARNING]   org.apache.maven.plugins:maven-release-plugin:3.1.1
[WARNING] 
[WARNING] Enable debug to see precisely which goals are not marked as thread-safe.
[WARNING] *****************************************************************
```

Seems that the release plugin should support threadsafe execution. Thanks!

---

**Affects:** 3.1.1

Reproduce the release with parallel execution enabled through .mvn/maven.config and confirm the warning for maven-release-plugin:3.1.1. Start by tracing which release goals are involved and how their thread-safety is declared. Done means the release plugin supports the parallel build without the non-thread-safe plugin warning.

Written by the indexing model from the issue text.