Fix "conn" leaking

[martelli]

On machines with many NICs, it can cause the golang runtime to run out of threads.

[pmazzini]

lgtm. Thanks for fixing it.

[codecov]

Codecov Report

Merging #351 into master will decrease coverage by 0.16%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #351      +/-   ##
==========================================
- Coverage   72.13%   71.96%   -0.17%     
==========================================
  Files          79       79              
  Lines        3994     3970      -24     
==========================================
- Hits         2881     2857      -24     
  Misses        954      954              
  Partials      159      159

Impacted Files	Coverage Δ
netboot/rtnetlink_linux.go	55.55% <100%> (+5.55%)	⬆️
dhcpv6/option_iaprefix.go	88.57% <0%> (-1.43%)	⬇️
dhcpv6/option_prefixdelegation.go	100% <0%> (ø)	⬆️
dhcpv6/option_nontemporaryaddress.go	100% <0%> (ø)	⬆️
dhcpv6/option_iaaddress.go	100% <0%> (ø)	⬆️

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[pmazzini]

Go tip linter check is failing but it is unrelated to this PR

https://travis-ci.org/insomniacslk/dhcp/builds/631842573

[pmazzini]

The error looks to be a golangci-lint issue: golangci/golangci-lint#896

I would go ahead and merge this PR but I can only do so if travis run was successful.

cc @insomniacslk

[insomniacslk]

@pmazzini i can force the merge. However I'd like a check of the return value of Close() before, and logging a warning if it fails, cc @martelli

[martelli]

I've refactored it to log any error messages.

[pmazzini]

lgtm