reconnect on transient network errors

[rberrelleza]

Proposed changes

• retry on the main loop for transient errors
• properly close ports during shutdown

[codecov]

Codecov Report

Merging #1148 into master will decrease coverage by 0.13%.
The diff coverage is 5.88%.

@@            Coverage Diff             @@
##           master    #1148      +/-   ##
==========================================
- Coverage   34.13%   34.00%   -0.14%     
==========================================
  Files          65       65              
  Lines        5467     5491      +24     
==========================================
+ Hits         1866     1867       +1     
- Misses       3398     3421      +23     
  Partials      203      203              

Impacted Files	Coverage Δ
cmd/up/up.go	3.91% <0.00%> (-0.06%)	⬇️
pkg/ssh/exec.go	0.00% <0.00%> (ø)
pkg/ssh/pool.go	53.70% <50.00%> (+0.87%)	⬆️

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