Support Distributed Mutations via GraphQL

[manishrjain]

Mutations via GraphQL which I introduced recently, won't work the same way as our bulk loader does. Because these mutations would need to go over the network; while our bulk loader, even in distributed state, does not. The general idea to do mutations is as follows:

Step 1: Network calls to assign Uids for all Xids.

Step 2: Distributed based on predicates, and multiple network calls to reach all corresponding servers. One RPC each server to load up the edges.

Step 3: Wait for success. If success, return or run the query part of GraphQL.
In case of failure, return to client; and let them retry. Later, we can add a retry loop here. Note that without transactions, some of these would have been applied; without any particular way to revert their changes (potential overwrites over data before this change was made). *

• The transactional behaviour would fall under distributed transactions, which we don't intend to support at least until v1.0. So, for now, we'll have this issue where a failure would result in some unapplied RDFs -- in general, until distributed transactions, we don't provide atomicity over multiple RDFs. Each RDF is applied atomically; and independently from others.

[manishrjain]

Distributed mutations now work.