add dns host provider lookup timeout

[MKrupauskas]

Resolves #142 by adding a 3s timeout for DNS lookups. This is better behavior than having the call get stuck indefinitely when the DNS resolver isn't reachable.

Adjusted/added unit tests to verify.

$ go test -run ^TestNewDNSHostProvider$
PASS
ok      github.com/go-zookeeper/zk      0.207s

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.60%. Comparing base (6585b41) to head (474e511).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #143      +/-   ##
==========================================
+ Coverage   78.35%   78.60%   +0.25%     
==========================================
  Files           8        8              
  Lines        1349     1365      +16     
==========================================
+ Hits         1057     1073      +16     
  Misses        201      201              
  Partials       91       91              

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