vendor: update cortex (#512)

Gopkg.lock

pkg/ingester/flush.go

pkg/ingester/stream.go

pkg/logproto/dep.go

pkg/util/conv.go

.../github.com/cortexproject/cortex/pkg/chunk/chunk_store.go

...b.com/cortexproject/cortex/pkg/chunk/encoding/bigchunk.go

...texproject/cortex/pkg/chunk/gcp/bigtable_object_client.go

...github.com/cortexproject/cortex/pkg/chunk/series_store.go

...ithub.com/cortexproject/cortex/pkg/chunk/storage/bytes.go

...ortexproject/cortex/pkg/chunk/storage/caching_fixtures.go

...xproject/cortex/pkg/chunk/storage/caching_index_client.go

...oject/cortex/pkg/chunk/storage/caching_index_client.pb.go

...oject/cortex/pkg/chunk/storage/caching_index_client.proto

...hub.com/cortexproject/cortex/pkg/chunk/storage/factory.go

...ub.com/cortexproject/cortex/pkg/ingester/client/compat.go

...com/cortexproject/cortex/pkg/ingester/client/cortex.pb.go

...com/cortexproject/cortex/pkg/ingester/client/cortex.proto

...ithub.com/cortexproject/cortex/pkg/ingester/client/fnv.go

...om/cortexproject/cortex/pkg/ingester/client/timeseries.go