Extend whelk to RDF dump feature with n-quads support

[niklasl]

Example usage:

cd importers
../gradlew jar
mkdir -p /tmp/libris-dumps
java -Dxl.secret.properties=../secret.properties.dev -jar build/libs/xlimporter.jar \
        lddbToNquads /tmp/libris-dumps/dev-bib-nq 4 --gzip bib

This is still WIP for various reasons:

• Compliation of importers failed on develop (possibly due to gradle bump). This is adressed in 5502f9e which we may want to cherry-pick if that's the way to go.
• We must to use a unique bnodes minter (e.g. doc-and-batch-count as prefix, or uuid:s) per conversion (a.k.a. "standardize apart"); the current nquads writer here will result in horrendous bnode smushing!
• I prefer to isolate trld-java API dependency a bit more (any may improve its public API further to do so).
• TRLD 0.3.0 includes proper n-quads support (and c14n, though we don't need it here); so we should either just bump trld-java, or backport that part (crucially fixes newlines in strings).
• We do some intricate cleanup in JsonLdToTrigSerializer which we may want to go about differently to reuse here (ideally of course fix the data in XL to prevent e.g. broken IRIs from ever entering).

[niklasl]

(Rebased on develop to account for switch to slf4j.)