Fix ByteBuf memory leaks, race conditions, corner case error handling

[nicmunroe]

This should fix the following:

• No more Netty ByteBuf reference counting memory leaks. If you saw any log messages that looked like LEAK: ByteBuf.release() was not called before it's garbage-collected then this should fix that.
• No more (rare) race conditions when processing ProxyRouterEndpoint requests that could lead to requests not being handled correctly.
• Better internal corner-case error handling for ProxyRouterEndpoint requests, leading to less log spam warnings.
• Improved logging when corner case errors do pop up. If a request is handled in an unexpected way and you have the trace ID from the response headers, then the logs should give you better insight into what happened (i.e. caller or downstream system dropping connection partway through a request).

[codecov-io]

Codecov Report

Merging #48 into master will decrease coverage by 0.24%.
The diff coverage is 74.18%.

@@             Coverage Diff              @@
##             master      #48      +/-   ##
============================================
- Coverage     90.61%   90.37%   -0.25%     
- Complexity     1899     1947      +48     
============================================
  Files           141      141              
  Lines          5573     5797     +224     
  Branches        726      765      +39     
============================================
+ Hits           5050     5239     +189     
- Misses          348      379      +31     
- Partials        175      179       +4

Impacted Files	Coverage Δ	Complexity Δ
...e/riposte/server/handler/RequestFilterHandler.java	100% <ø> (+2.04%)	21 <0> (+1)	⬆️
...oste/metrics/codahale/CodahaleMetricsListener.java	100% <100%> (ø)	39 <0> (ø)	⬇️
...poste/server/handler/RequestInfoSetterHandler.java	100% <100%> (+9.37%)	20 <10> (+4)	⬆️
...r/handler/NonblockingEndpointExecutionHandler.java	100% <100%> (+5.35%)	28 <13> (+4)	⬆️
...n/java/com/nike/riposte/util/AsyncNettyHelper.java	100% <100%> (ø)	48 <0> (+1)	⬆️
...nike/riposte/server/http/impl/RequestInfoImpl.java	100% <100%> (ø)	69 <4> (+5)	⬆️
...rver/handler/IncompleteHttpCallTimeoutHandler.java	100% <100%> (ø)	7 <4> (+2)	⬆️
...iposte/server/http/ProxyRouterProcessingState.java	94.11% <100%> (+7.45%)	13 <2> (+2)	⬆️
...poste/server/handler/ExceptionHandlingHandler.java	100% <100%> (+4.61%)	27 <0> (+2)	⬆️
...erver/handler/ChannelPipelineFinalizerHandler.java	100% <100%> (+7.69%)	41 <20> (+23)	⬆️
... and 13 more

---

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 6b5afbf...a74fdfa. Read the comment docs.