nghttpx: Workaround for std::make_shared bug in Xcode7, 7.1, and 7.2

std::make_shared in Xcode 7, 7.1, and 7.2 does not perform
value-initialization, and causes undefined behaviour if struct does
not have user defined default constructor.  This workaround explicitly
defines user defined default constructor, and initializes values.

src/shrpx_config.h

@@ -649,6 +649,15 @@ struct RouterConfig {
 };
 
 struct DownstreamConfig {
+  DownstreamConfig()
+      : timeout{},
+        addr_group_catch_all{0},
+        connections_per_host{0},
+        connections_per_frontend{0},
+        request_buffer_size{0},
+        response_buffer_size{0},
+        family{0} {}
+
   struct {
     ev_tstamp read;
     ev_tstamp write;

src/shrpx_worker.cc

@@ -35,8 +35,6 @@
 #include "shrpx_client_handler.h"
 #include "shrpx_http2_session.h"
 #include "shrpx_log_config.h"
-#include "shrpx_connect_blocker.h"
-#include "shrpx_live_check.h"
 #include "shrpx_memcached_dispatcher.h"
 #ifdef HAVE_MRUBY
 #include "shrpx_mruby.h"
@@ -186,8 +184,6 @@ void Worker::replace_downstream_config(
     dst = std::make_shared<DownstreamAddrGroup>();
     dst->pattern = src.pattern;
 
-    // TODO for some reason, clang-3.6 which comes with Ubuntu 15.10
-    // does not value initialize with std::make_shared.
     auto shared_addr = std::make_shared<SharedDownstreamAddr>();
 
     shared_addr->addrs.resize(src.addrs.size());

src/shrpx_worker.h

@@ -46,6 +46,8 @@
 #include "shrpx_downstream_connection_pool.h"
 #include "memchunk.h"
 #include "shrpx_ssl.h"
+#include "shrpx_live_check.h"
+#include "shrpx_connect_blocker.h"
 
 using namespace nghttp2;
 
@@ -53,7 +55,6 @@ namespace shrpx {
 
 class Http2Session;
 class ConnectBlocker;
-class LiveCheck;
 class MemcachedDispatcher;
 struct UpstreamAddr;
 class ConnectionHandler;
@@ -126,6 +127,9 @@ struct WeightedPri {
 };
 
 struct SharedDownstreamAddr {
+  SharedDownstreamAddr()
+      : next{0}, http1_pri{}, http2_pri{}, affinity{AFFINITY_NONE} {}
+
   std::vector<DownstreamAddr> addrs;
   // Bunch of session affinity hash.  Only used if affinity ==
   // AFFINITY_IP.
@@ -156,6 +160,8 @@ struct SharedDownstreamAddr {
 };
 
 struct DownstreamAddrGroup {
+  DownstreamAddrGroup() : retired{false} {};
+
   ImmutableString pattern;
   std::shared_ptr<SharedDownstreamAddr> shared_addr;
   // true if this group is no longer used for new request.  If this is