client: Implement command timeouts

[foxcpp]

Closes #88.

[codecov]

Codecov Report

Merging #115 into master will increase coverage by 0.37%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #115      +/-   ##
==========================================
+ Coverage   62.09%   62.46%   +0.37%     
==========================================
  Files           8        8              
  Lines        1108     1119      +11     
==========================================
+ Hits          688      699      +11     
  Misses        312      312              
  Partials      108      108              

Impacted Files	Coverage Δ
client.go	67.70% <100.00%> (+1.14%)	⬆️

---

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 1918b06...f180265. Read the comment docs.

[corny]

Please also write a simple test with a stub server to simulate the timeout.

[kayrus]

Any updates on this PR?

[foxcpp]

Any updates on this PR?

I have a lot of IRL things to do. Feel free to finish this PR if you're interested.

[foxcpp]

I renamed DotTimeout into SubmissionTimeout and also rebased PR onto upstream master so tests could run (there is new vet rule in Go 1.15 that failed)

[emersion]

LGTM, thanks!