v3 proto: end-to-end test for select & insert
v3 proto: VCursor interface for lookup queries

Vindexes need to be able to execute queries to access
their associated tables. So, we add a VCursor interface
to all functions so the vindexes can use them. VTGate
is responsible for creating a VCursor compliant variable
and pass it down.
v3 proto: execution for Unsharded plans
v3 proto: vcursor and insert half done

Changed router to implement and use requestContext,
which also satisfies the planbuilder.VCursor interface.
Changed insert to do the right thing for the primary vindex.
v3 proto: end-to-end test for insert primary
v3 proto: lookup hash unique index and handling of non-primary indexes for inserts
v3 proto: insert router unit tests
v3 proto: insert end-to-end tests
Merge pull request #213 from youtube/vtgate

Vtgate