NEXUS-6213: Backported fix to 2.7.x

Squashed commits:
3f04733b41b5e759d056760f6634f3ce46fe6872
8be363a440ff20a5aa3a2682a67b0245d935833e
19f85dc6261c82f9fdf78b9ccfded7c32b17926f
d6846c3eed0cd0f21865c9bf9ca575d788e7b97f

Original PR
https://github.com/sonatype/nexus-oss/pull/313
NEXUS-6220: Backport to 2.7.x

Commits:
9000d5ee7462ef6948323feda218592dedd18674
91e10a8ee912775e8479ad98a45a79d1f386f276
bc33bff36cfab6ab51adbf2e781ab0464dde70a4
bf31deb2afd84522e9a2206952b04b9b39000119

Original PR:
https://github.com/sonatype/nexus-oss/pull/315
NEXUS-6230: Backport to 2.7.x

Nullcheck was missing for listedGav
As the folder might contain maven metadata and other stuff
that M2GavCalculator cannot interpret, and will return
null.
NEXUS-6242: Backport to Nexus 2.7.x

How parser is looked up, one change from this commit
https://github.com/sonatype/nexus-oss/commit/7cab8b575936a521889ddf194ac34187a78a6031#diff-0
NEXUS-6242: Backport to Nexus 2.7.x

Commit
95f212c2e946dd6d6f9c8a1a1671ee180bd78d0d
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
Merge pull request #332 from sonatype/nexus-2.7.x-NEXUS-6242

[2.7.x] NEXUS-6242: Backport to Nexus 2.7.x

merging based on +1s
update version to 2.7.2-SNAPSHOT
update @since to 2.7.2
Merge remote-tracking branch 'origin/nexus-2.7.x' into nexus-2.7.x-nx-client-changes
Fixing since tags.

Some of them were backported.
Some of them originates from security 2.8, when
the project was on it's own repository. Using
already used designation for those, prepending
project name to version to avoid clash.

Note: Security was at version 2.8 when it got
squashed into Nexus OSS repository. Today,
Nexus is at version 2.8, but we had leftovers
of some security components having no designation
and hence, leading to confusion.
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
More since fixes.
Merge pull request #317 from sonatype/nexus-2.7.x-NEXUS-6213-site-xml

[2.7.x] NEXUS-6213: Backported fix to 2.7.x
NEXUS-6220: Adjust since tags
Merge pull request #318 from sonatype/nexus-2.7.x-NEXUS-6220-conn-reuse

[2.7.x] NEXUS-6220: Backport to 2.7.x
Merge pull request #331 from sonatype/nexus-2.7.x-NEXUS-6230

[2.7.x] NEXUS-6230: Backport to 2.7.x