Checkum policy blocks prefix list

Fixes
http://issues.sonatype.org/browse/NXCM-5188

Multiple fixes:
* NPE fixed that is apparent in log in the issue (is general, not prefix file related!)
* Implemented a fix: override per-request the ChecksumPolicy in Maven repositories
* making use of the core change in `RemotePrefixFileStrategy`, added UT
* added comments to remove temptation (like I had)

CI
https://bamboo.zion.sonatype.com/browse/NXO-OSSF1-1