Migrate from dep to go module

Signed-off-by: Madhu Rajanna <madhupr007@gmail.com>

vendor/github.com/modern-go/concurrent/.gitignore

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+/coverage.txt

vendor/github.com/modern-go/concurrent/.travis.yml

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+language: go
+
+go:
+  - 1.8.x
+  - 1.x
+
+before_install:
+  - go get -t -v ./...
+
+script:
+  - ./test.sh
+
+after_success:
+  - bash <(curl -s https://codecov.io/bash)

vendor/github.com/modern-go/concurrent/README.md

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+# concurrent
+
+[![Sourcegraph](https://sourcegraph.com/github.com/modern-go/concurrent/-/badge.svg)](https://sourcegraph.com/github.com/modern-go/concurrent?badge)
+[![GoDoc](http://img.shields.io/badge/go-documentation-blue.svg?style=flat-square)](http://godoc.org/github.com/modern-go/concurrent)
+[![Build Status](https://travis-ci.org/modern-go/concurrent.svg?branch=master)](https://travis-ci.org/modern-go/concurrent)
+[![codecov](https://codecov.io/gh/modern-go/concurrent/branch/master/graph/badge.svg)](https://codecov.io/gh/modern-go/concurrent)
+[![rcard](https://goreportcard.com/badge/github.com/modern-go/concurrent)](https://goreportcard.com/report/github.com/modern-go/concurrent)
+[![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://raw.githubusercontent.com/modern-go/concurrent/master/LICENSE)
+
+* concurrent.Map: backport sync.Map for go below 1.9
+* concurrent.Executor: goroutine with explicit ownership and cancellable
+
+# concurrent.Map
+
+because sync.Map is only available in go 1.9, we can use concurrent.Map to make code portable
+
+```go
+m := concurrent.NewMap()
+m.Store("hello", "world")
+elem, found := m.Load("hello")
+// elem will be "world"
+// found will be true
+```
+
+# concurrent.Executor
+
+```go
+executor := concurrent.NewUnboundedExecutor()
+executor.Go(func(ctx context.Context) {
+    everyMillisecond := time.NewTicker(time.Millisecond)
+    for {
+        select {
+        case <-ctx.Done():
+            fmt.Println("goroutine exited")
+            return
+        case <-everyMillisecond.C:
+            // do something
+        }
+    }
+})
+time.Sleep(time.Second)
+executor.StopAndWaitForever()
+fmt.Println("executor stopped")
+```
+
+attach goroutine to executor instance, so that we can
+
+* cancel it by stop the executor with Stop/StopAndWait/StopAndWaitForever
+* handle panic by callback: the default behavior will no longer crash your application

vendor/github.com/modern-go/concurrent/test.sh

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+#!/usr/bin/env bash
+
+set -e
+echo "" > coverage.txt
+
+for d in $(go list ./... | grep -v vendor); do
+    go test -coverprofile=profile.out -coverpkg=github.com/modern-go/concurrent $d
+    if [ -f profile.out ]; then
+        cat profile.out >> coverage.txt
+        rm profile.out
+    fi
+done