NEXUS-6164: Backport to Nexus 2.7.x

Expose SSL cert check lax options

Original PR
https://github.com/sonatype/nexus-oss/pull/281
NEXUS-6182: Backport to Nexus 2.7.x

Make client multi-thread capable

Original PR
https://github.com/sonatype/nexus-oss/pull/322
update @since to 2.7.2
Merge remote-tracking branch 'origin/nexus-2.7.x' into nexus-2.7.x-nx-client-changes
Revert "update @since to 2.7.2"

This reverts commit b86b5fdb55268b8347effb9b3287f53ac3bdbf33.
Merge remote-tracking branch 'origin/nexus-2.7.x' into nexus-2.7.x-nx-client-changes
Adjust since tags
Merge pull request #335 from sonatype/nexus-2.7.x-nx-client-changes

[2.7.x] Backport Nexus Client changes.