fix(dialogflow): use for await instead of promise.all

[chentsulin]

use for await instead of promise.all

fix #844

[codecov-commenter]

Codecov Report

Merging #851 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #851   +/-   ##
=======================================
  Coverage   83.23%   83.23%           
=======================================
  Files         108      108           
  Lines        5647     5647           
  Branches     1367     1422   +55     
=======================================
  Hits         4700     4700           
  Misses        924      924           
  Partials       23       23           

Impacted Files	Coverage Δ
packages/bottender-dialogflow/src/index.ts	95.45% <100.00%> (ø)

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