refactor: make Connection#makeRequest use async/await

[arthurschreiber]

This refactors Connection#makeRequest to be an async function, and to use async/await to make the code flow more explicit.

I pulled in all the logic from enter handlers of the SENT_CLIENT_REQUEST and SENT_ATTENTION states, so Connection#makeRequest contains now all the logic of sending a request, handling cancellation, and reading the response.

[codecov]

Codecov Report

❌ Patch coverage is 71.20419% with 55 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.91%. Comparing base (b78df14) to head (903c1c5).
⚠️ Report is 75 commits behind head on master.

Files with missing lines	Patch %	Lines
src/connection.ts	71.20%	45 Missing and 10 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1492      +/-   ##
==========================================
+ Coverage   79.83%   79.91%   +0.07%     
==========================================
  Files          92       91       -1     
  Lines        4692     4765      +73     
  Branches      871      886      +15     
==========================================
+ Hits         3746     3808      +62     
+ Misses        673      667       -6     
- Partials      273      290      +17     

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