New upsert block

[mangalaman93]

Related to #3197
Fixes #3059

Example upsert operation -

   upsert {
      query {
        me(func: eq(email, "aman@dgraph.io")) {
          v as uid
        }
      }
      mutation {
          set {
            uid(v) <name> "Aman" .
            uid(v) <email> "aman@dgraph.io" .
          }
      }
    }

Probably in the next version -

• Add support for multi valued variables
• Support for val function
• Add support for @if
• Fuzz tests

Changes in other repos -

• Add tests in dgo repo Add test for Upsert block dgo#68
• Python Client Add support for Upsert Block and bump version to 1.2.0 pydgraph#74
• Java Client Add support for upsert block dgraph4j#88
• JS Client hypermodeinc/dgraph-js@2156353

Todo

• Add lots of tests for RDF and JSON
• Add documentation (docs.dgraph.io)
• Add support for tracing
• JSON format for upsert block
• Add V(3) logging
• Support for uid function
• Fragments in upsert block
• Two query blocks inside upsert block?
• no query block inside upsert block?
• no mutation block inside upsert block?
• no variable in query block?
• support for blank nodes in mutation block?
• Ensure upsert only generates 1 UID

This change is 

[MichaelJCompton]

OK

I'll talk to others here this week to understand my problem - looks like everyone else is ok with it.

Reviewable status: 12 of 15 files reviewed, 35 unresolved discussions (waiting on @danielmai, @gitlw, @golangcibot, @manishrjain, and @martinmr)

[MichaelJCompton]

Reviewable status: 12 of 15 files reviewed, 35 unresolved discussions (waiting on @danielmai, @gitlw, @golangcibot, @manishrjain, and @martinmr)

[manishrjain]

@MichaelJCompton : I can answer that question.