stop using raw libuv API

[StefanKarpinski]

• use Timer instead of libuv timer API
• use FDWatcher instead of libuv poll API

[codecov]

Codecov Report

Merging #151 (f5a0ba9) into master (5f1509d) will decrease coverage by 1.82%.
The diff coverage is 92.68%.

❗ Current head f5a0ba9 differs from pull request most recent head 2eb0491. Consider uploading reports for the commit 2eb0491 to get more accurate results

@@            Coverage Diff             @@
##           master     #151      +/-   ##
==========================================
- Coverage   92.77%   90.94%   -1.83%     
==========================================
  Files           5        5              
  Lines         498      464      -34     
==========================================
- Hits          462      422      -40     
- Misses         36       42       +6     

Impacted Files	Coverage Δ
src/Curl/Multi.jl	92.59% <91.17%> (-3.28%)	⬇️
src/Curl/Curl.jl	100.00% <100.00%> (ø)
src/Curl/utils.jl	50.00% <100.00%> (-47.57%)	⬇️
src/Curl/Easy.jl	94.09% <0.00%> (-0.24%)	⬇️
src/Downloads.jl	83.00% <0.00%> (+0.17%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5f1509d...2eb0491. Read the comment docs.

[KristofferC]

I think this broke Pkg: JuliaLang/Pkg.jl#2802 (comment)