[misc] Fix robustness issues in the DeleteServlet

[sdumitriu]

• Reference cycles, e.g. two resources holding mutual RECURSIVE_DELETE links to each other, made the referrer traversal recurse until the stack overflowed; each node's referrers are now only chased once per phase
• Deleting a link's own definition consulted that definition's onDelete policy, although the policy describes what to do when the link's target is deleted; a RECURSIVE_DELETE definition would take every linking resource down with it. A link cannot outlive its definition, so the link itself is now removed without consulting the policy
• A hard link with the (illegal) IGNORE policy, or an unrecognized policy value, passed the conflict check untouched and then blew up at commit time with a ReferentialIntegrityException reported as a generic 500; such links are now removed, with a warning for the illegal policy
• A link with an unresolvable definition crashed the whole request; the policy now falls back to only removing the link, with a warning
• One node failing to describe itself made the whole 409 explanation collapse into 'referenced by unknown item(s)'; failures are now counted among the 'other' items instead of discarding the rest of the list
• After sending the 409 explanation, control still fell through to the removal loop and session.save(), which would persist any unrelated pending changes; refusals now skip the removal phase entirely
• A versionable ancestor checked out for the removal of one node could be removed by the same operation as a referrer, making the final checkin crash on the missing path; checkins are now path-based and skip deleted nodes
• A request for a non-JCR resource crashed with a NullPointerException instead of answering 404
• Also removed the resolver ThreadLocal and the mid-analysis save it existed for, which saved an empty transient state