refactor: drop usage of _.union

[JustinBeckwith]

This replaces _.union with a native Array.from(new Set(array.concat(array))) kinda thing.

[murgatroid99]

The Mac test result is showing several unusual errors under Node 4, but it's possible that they are just async errors propagated after the initial test timeout:

  1) Async functionality should not hang:
     Error: Timeout of 5000ms exceeded. For async tests and hooks, ensure "done()" is called; if returning a Promise, ensure it resolves.
  2) Async functionality "after all" hook:
     Uncaught Error: 2 UNKNOWN: Failed to create subchannel
      at Object.exports.createStatusError (packages/grpc-native-core/src/common.js:41:288)
      at Object.onReceiveStatus (packages/grpc-native-core/src/client_interceptors.js:421:555)
      at InterceptingListener._callNext (packages/grpc-native-core/src/client_interceptors.js:296:289)
      at InterceptingListener.onReceiveStatus (packages/grpc-native-core/src/client_interceptors.js:311:141)
      at packages/grpc-native-core/src/client_interceptors.js:375:267
  3) channel close should succeed silently:
     AssertionError: Got unwanted exception..
      at _throws (assert.js:356:5)
      at Function.assert.doesNotThrow (assert.js:374:3)
      at Context.<anonymous> (packages/grpc-native-core/test/channel_test.js:105:14)
  4) channel close should succeed silently:
     AssertionError: Got unwanted exception..
      at _throws (assert.js:356:5)
      at Function.assert.doesNotThrow (assert.js:374:3)
      at Context.<anonymous> (packages/grpc-native-core/test/channel_test.js:105:14)
  5) call startBatch with metadata should succeed with a map of strings to string arrays:
     Uncaught Error: The async function encountered an error
      at Error (native)
  6) call startBatch with metadata should succeed with a map of strings to buffer arrays:
     Uncaught Error: The async function encountered an error
      at Error (native)