Crash in Envoy::Http::AsyncStreamImpl::closeRemote on shutdown

Seeing the following crash running off 38a4d86407668612ccbc4cba65d11be6cbd81b32:

```
2019-08-20_22:24:17.36421 [2019-08-20 22:24:17.364][374905][warning][main] [external/envoy/source/server/server.cc:469] caught SIGTERM
2019-08-20_22:24:17.36423 [2019-08-20 22:24:17.364][374905][info][main] [external/envoy/source/server/server.cc:574] shutting down server instance
2019-08-20_22:24:17.36424 [2019-08-20 22:24:17.364][374905][info][main] [external/envoy/source/server/server.cc:528] main dispatch loop exited
2019-08-20_22:24:17.36959 [2019-08-20 22:24:17.369][376231][critical][backtrace] [bazel-out/k8-opt/bin/external/envoy/source/server/_virtual_includes/backtrace_lib/server/backtrace.h:81] Caught Segmentation f
ault, suspect faulting address 0x0
2019-08-20_22:24:17.36962 [2019-08-20 22:24:17.369][376231][critical][backtrace] [bazel-out/k8-opt/bin/external/envoy/source/server/_virtual_includes/backtrace_lib/server/backtrace.h:69] Backtrace (use tools/
stack_decode.py to get line numbers):
2019-08-20_22:24:17.37594 [2019-08-20 22:24:17.375][376231][critical][backtrace] [bazel-out/k8-opt/bin/external/envoy/source/server/_virtual_includes/backtrace_lib/server/backtrace.h:73] #0: __restore_rt [0x7
f1f290895d0]
2019-08-20_22:24:17.39615 [2019-08-20 22:24:17.396][376231][critical][backtrace] [bazel-out/k8-opt/bin/external/envoy/source/server/_virtual_includes/backtrace_lib/server/backtrace.h:73] #1: Envoy::Http::AsyncStreamImpl::closeRemote() [0x8f0b46]
2019-08-20_22:24:17.40303 [2019-08-20 22:24:17.402][376231][critical][backtrace] [bazel-out/k8-opt/bin/external/envoy/source/server/_virtual_includes/backtrace_lib/server/backtrace.h:73] #2: Envoy::Http::AsyncStreamImpl::encodeData() [0x8f5148]
2019-08-20_22:24:17.41097 [2019-08-20 22:24:17.410][376231][critical][backtrace] [bazel-out/k8-opt/bin/external/envoy/source/server/_virtual_includes/backtrace_lib/server/backtrace.h:73] #3: Envoy::Http::Utility::sendLocalReply() [0xa05ff8]
2019-08-20_22:24:17.41454 [2019-08-20 22:24:17.414][376231][critical][backtrace] [bazel-out/k8-opt/bin/external/envoy/source/server/_virtual_includes/backtrace_lib/server/backtrace.h:73] #4: Envoy::Http::AsyncStreamImpl::sendLocalReply() [0x8f0e69]
2019-08-20_22:24:17.41906 [2019-08-20 22:24:17.419][376231][critical][backtrace] [bazel-out/k8-opt/bin/external/envoy/source/server/_virtual_includes/backtrace_lib/server/backtrace.h:73] #5: Envoy::Router::Filter::onUpstreamAbort() [0x8f71c3]
2019-08-20_22:24:17.42254 [2019-08-20 22:24:17.422][376231][critical][backtrace] [bazel-out/k8-opt/bin/external/envoy/source/server/_virtual_includes/backtrace_lib/server/backtrace.h:73] #6: Envoy::Router::Filter::onUpstreamReset() [0x902bee]
2019-08-20_22:24:17.42603 [2019-08-20 22:24:17.425][376231][critical][backtrace] [bazel-out/k8-opt/bin/external/envoy/source/server/_virtual_includes/backtrace_lib/server/backtrace.h:73] #7: Envoy::Router::Filter::UpstreamRequest::onResetStream() [0x902fb2]
2019-08-20_22:24:17.42950 [2019-08-20 22:24:17.429][376231][critical][backtrace] [bazel-out/k8-opt/bin/external/envoy/source/server/_virtual_includes/backtrace_lib/server/backtrace.h:73] #8: Envoy::Http::Http1::ClientConnectionImpl::onResetStream() [0x95f30d]
2019-08-20_22:24:17.43457 [2019-08-20 22:24:17.434][376231][critical][backtrace] [bazel-out/k8-opt/bin/external/envoy/source/server/_virtual_includes/backtrace_lib/server/backtrace.h:73] #9: Envoy::Http::CodecClient::onEvent() [0x8e27a4]
2019-08-20_22:24:17.44230 [2019-08-20 22:24:17.442][376231][critical][backtrace] [bazel-out/k8-opt/bin/external/envoy/source/server/_virtual_includes/backtrace_lib/server/backtrace.h:73] #10: Envoy::Network::ConnectionImpl::raiseEvent() [0x753b25]
2019-08-20_22:24:17.44620 [2019-08-20 22:24:17.446][376231][critical][backtrace] [bazel-out/k8-opt/bin/external/envoy/source/server/_virtual_includes/backtrace_lib/server/backtrace.h:73] #11: Envoy::Network::ConnectionImpl::closeSocket() [0x753045]
2019-08-20_22:24:17.45422 [2019-08-20 22:24:17.454][376231][critical][backtrace] [bazel-out/k8-opt/bin/external/envoy/source/server/_virtual_includes/backtrace_lib/server/backtrace.h:73] #12: Envoy::Network::ConnectionImpl::close() [0x75489d]
2019-08-20_22:24:17.45843 [2019-08-20 22:24:17.458][376231][critical][backtrace] [bazel-out/k8-opt/bin/external/envoy/source/server/_virtual_includes/backtrace_lib/server/backtrace.h:73] #13: Envoy::Http::Http1::ConnPoolImpl::~ConnPoolImpl() [0x88abf5]
2019-08-20_22:24:17.46249 [2019-08-20 22:24:17.462][376231][critical][backtrace] [bazel-out/k8-opt/bin/external/envoy/source/server/_virtual_includes/backtrace_lib/server/backtrace.h:73] #14: Envoy::Http::Http1::ProdConnPoolImpl::~ProdConnPoolImpl() [0x88ad00]
2019-08-20_22:24:17.46651 [2019-08-20 22:24:17.466][376231][critical][backtrace] [bazel-out/k8-opt/bin/external/envoy/source/server/_virtual_includes/backtrace_lib/server/backtrace.h:73] #15: Envoy::Upstream::ConnPoolMap<>::clearActivePools() [0x78500e]
2019-08-20_22:24:17.46989 [2019-08-20 22:24:17.469][376231][critical][backtrace] [bazel-out/k8-opt/bin/external/envoy/source/server/_virtual_includes/backtrace_lib/server/backtrace.h:73] #16: Envoy::Upstream::ConnPoolMap<>::~ConnPoolMap() [0x78c2e5]
2019-08-20_22:24:17.47316 [2019-08-20 22:24:17.473][376231][critical][backtrace] [bazel-out/k8-opt/bin/external/envoy/source/server/_virtual_includes/backtrace_lib/server/backtrace.h:73] #17: std::_Sp_counted_ptr_inplace<>::_M_dispose() [0x78c428]
2019-08-20_22:24:17.47640 [2019-08-20 22:24:17.476][376231][critical][backtrace] [bazel-out/k8-opt/bin/external/envoy/source/server/_virtual_includes/backtrace_lib/server/backtrace.h:73] #18: std::_Hashtable<>::clear() [0x784e4a]
2019-08-20_22:24:17.47980 [2019-08-20 22:24:17.479][376231][critical][backtrace] [bazel-out/k8-opt/bin/external/envoy/source/server/_virtual_includes/backtrace_lib/server/backtrace.h:73] #19: Envoy::Upstream::ClusterManagerImpl::ThreadLocalClusterManagerImpl::~ThreadLocalClusterManagerImpl() [0x789c2b]
2019-08-20_22:24:17.48601 [2019-08-20 22:24:17.485][376231][critical][backtrace] [bazel-out/k8-opt/bin/external/envoy/source/server/_virtual_includes/backtrace_lib/server/backtrace.h:73] #20: Envoy::ThreadLoc
al::InstanceImpl::shutdownThread() [0x70873a]
2019-08-20_22:24:17.48990 [2019-08-20 22:24:17.489][376231][critical][backtrace] [bazel-out/k8-opt/bin/external/envoy/source/server/_virtual_includes/backtrace_lib/server/backtrace.h:73] #21: Envoy::Server::W
orkerImpl::threadRoutine() [0x746d49]
2019-08-20_22:24:17.49403 [2019-08-20 22:24:17.493][376231][critical][backtrace] [bazel-out/k8-opt/bin/external/envoy/source/server/_virtual_includes/backtrace_lib/server/backtrace.h:73] #22: Envoy::Thread::T
hreadImplPosix::ThreadImplPosix()::{lambda()#1}::_FUN() [0xc879c5]
2019-08-20_22:24:17.49410 [2019-08-20 22:24:17.494][376231][critical][backtrace] [bazel-out/k8-opt/bin/external/envoy/source/server/_virtual_includes/backtrace_lib/server/backtrace.h:73] #23: start_thread [0x
7f1f29081dd5]
2019-08-20_22:24:17.49412 AsyncClient 0x4981808, stream_id_: 1003858367269869698
2019-08-20_22:24:17.49413 &stream_info_:
2019-08-20_22:24:17.49413 StreamInfoImpl 0x4981960, protocol_: 1, response_code_: null, response_code_details_: upstream_reset_before_response_started{connection termination}, health_check_request_: 0, rout
e_name_:
```

Not 100% sure that this was introduced with our recent update, as we've been seeing crashes with similar traces on older versions.