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

Merging #1492 (903c1c5) into master (b78df14) will increase coverage by 0.07%.
The diff coverage is 71.20%.

@@            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     

Impacted Files	Coverage Δ
src/connection.ts	64.11% <71.20%> (+2.07%)	⬆️

... and 4 files with indirect coverage changes