Handle tombstone resources for deletions in LDES feed

[abeforgit]

• fix(ldes): handle Tombstone types
• fix(ldes): cleaner method of selecting tombstones

Overview

the LDES feed uses the Tombstone type to indicate deleted items.
It swaps out the subject's type for Tombstone, and adds the old type under the
formerType predicate. It's a clean mechanism, we just have to make sure to
capture the tombstone types to overwrite the old data.

connected issues and PRs:

Setup

this will of course only work on new ldes data, so you have to reset your ldes
state for it to pick up on all the deletions

here's how to reset your ldes state without nuking your db:

PREFIX ext: <http://mu.semte.ch/vocabularies/ext/>
DELETE {
  GRAPH ?g {
    ?s ?p ?v.
  }
} WHERE { 
  GRAPH ?g { 
    ?s a ext:LDESClientState; 
        ?p ?v. 
  } 
}

this will cause the client to do a full ingest on the next tick, so make sure your BYPASS_MU_AUTH is set accordingly

How to test/reproduce

Challenges/uncertainties

Checks PR readiness

• changelog
• no new deprecations

[piemonkey]

Seems to work for me once I've actually let ldes-client catch up.