REVIEW: P2 agent temp directory related fixes

https://issues.sonatype.org/browse/NXCM-4485
https://issues.sonatype.org/browse/NXCM-4660
https://issues.sonatype.org/browse/NEXUS-5225

* Put all temporary directories under sonatype-work/nexus/tmp/nexus-p2-bridge-plugin/tmp
* Drop p2-agent-* directories after usage