Prevent multiple calls to Server.shutdown()

[nicmunroe]

No description provided.

[codecov-io]

Codecov Report

Merging #118 into master will decrease coverage by 0.05%.
The diff coverage is 70%.

@@             Coverage Diff              @@
##             master     #118      +/-   ##
============================================
- Coverage     91.48%   91.43%   -0.06%     
- Complexity     2344     2345       +1     
============================================
  Files           161      161              
  Lines          6730     6735       +5     
  Branches        889      889              
============================================
+ Hits           6157     6158       +1     
- Misses          381      383       +2     
- Partials        192      194       +2

Impacted Files	Coverage Δ	Complexity Δ
.../src/main/java/com/nike/riposte/server/Server.java	74.31% <70%> (+2.19%)	15 <7> (+4)	⬆️
...mpipeline/DownstreamIdleChannelTimeoutHandler.java	85% <0%> (-5%)	4% <0%> (-1%)
...ient/asynchttp/netty/StreamingAsyncHttpClient.java	73.33% <0%> (-0.61%)	49% <0%> (-1%)
...r/handler/ProxyRouterEndpointExecutionHandler.java	60.86% <0%> (-0.32%)	26% <0%> (-1%)

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