Run maven with more than default single thread (--threads=0.5C)

See tail of parent task where we made commits to hbase-personality trying to make maven run more furiously on nightly builds by upping the maven thread count (the --threads flag ... see https://cwiki.apache.org/confluence/display/MAVEN/Parallel+builds+in+Maven+3). The effort was abandoned because it unsettled the build. This issue separates messing with the maven threads flag from the broader topic the parent covers.