NEXUS-5673: Logspam in shadows.

Handling at top level cases like:
* this shadow is out of service, no need to dive in at all
* INFEx silenced to DEBUG (with logging of event)
* Unsupported operation and illegal operation silenced to DEBUG (with logging of event)
* improved logging in general
NEXUS-5673: Logspam in shadows.

Layout converter maven shadow (that serves as basis for
M1-to-M2 and M2-to-M1 reposes) got extra catch that
simply ignores ItemNotFOundEx when removing links.
NEXUS-5673: Catching INFex only when link is not found

When link is not found, as in related issue, we still
let the empty parent cleanup to happen.
NEXUS-5673: No code change, just issue refs
NEXUS-5673: No code change, tabs2spaces
Merge pull request #822 from sonatype/nexus-5673-logspam-in-shadows

[REVIEW] NEXUS-5673: logspam in shadows