NXCM-5188: NPE fix

In the issue attached log, it's clearly visible that validation
did reject the item (policy=Strict but no hashes were present).
Later, an NPE is thrown, as the cleanup method is not protected
from this case.

In all other case it would work as contentValid would be
TRUE for all of those. This is the only branch in logic
where it is FALSE, and the remoteHash is null too.

In other words, cleanup is needed when content is
invalid, but hashes arrived into cache.
NXCM-5188: Added checksum relax possibility and using it in AR strategy

Added simple "per request" ChecksumPolicy override feature to maven
specific repositories (as ChecksumPolicy itself is maven specific).
Using it one can "relax" the policy for single request.

Making use of it in RemotePrefixFileStrategy, to relax
the policy if its STRICT to one step below (STRICT_IF_EXISTS).

Added UT that validates the change. UT will fail if relevant
changes are commented out from RemotePrefixFileStrategy,
NXCM-5188: Removed temptation by adding inline explanation

I was tempted to change these methods, but finally figured
it's completely unneeded.

Comment explains why.
NXCM-5188: Typo in comment
NXCM-5188: Checksum policy set to IGNORE for prefix requests

As the transport integrity here has small chances to slip in
as prefix file will be processed, and hence, checked/validated
Merge pull request #806 from sonatype/nxcm-5188-checkum-policy-blocks-prefix-list

Checkum policy blocks prefix list