Parallel build thread-safe

[Karlinator]

Just a general request to make the plugin thread-safe, or mark it as such if it already is.

Ref: https://cwiki.apache.org/confluence/display/MAVEN/Parallel+builds+in+Maven+3

This isn't a huge issue, but faster builds are always nice. This might be as simple as checking and confirming, or require major rewrites to implement; I haven't checked.