NXCM-4861: Fixing the ArtifactStoreHelper.storeItemWithChecksums method

In some cases, current imple can result in remote item download
for some proxy configuration, like itemAgingActive=false used by
procurement.

Fixing it, to enforce "localOnly" flag. Basically, same code
is now executed as in case of cache (where also store+retrieve)
happens, see:
org.sonatype.nexus.proxy.repository.AbstractProxyRepository#doCacheItem

Other minor cleanup (mostly javadoc) done.

Added UT that verifies that maven repo now receives a retrieve
with localOnly=true flag.