Add feature to download the whole graph

[bigerl]

see https://www.w3.org/TR/2013/REC-sparql11-http-rdf-update-20130321/#http-get

implement this option (without sending the

GET /rdf-graph-store?default HTTP/1.1
Host: example.com
Accept: text/turtle; charset=utf-8

• We don't want to build the full file in memory before we serialize it. Use HTTP/1.1 chunked transfer encoding or similar HTTP/2 mode (see also https://stiffstream.com/en/docs/restinio/0.6/responsebuilder.html#chunked-transfer-encoding-output-builder).
• support ntriple (requested via content type if I got the w3 specifications right)
• add option to use gzip for the stream
• support turtle (first without using prefixes but with subject and predicate groups)
• consider to scan all stored resources to find good prefixes (consider using Tessil's HAT trie for that (see also their blogpost at https://tessil.github.io/2017/06/22/hat-trie.html )

[bigerl]

@LukasKerk and @Shaheen47
after you finished the work on your current tasks you can start working on this one together