nghttpx: Make stream timeout disable by default

It might be useful to clean the unused stream out to make up the room
for new streams.  On the other hand, proxy should maintain the
connection between upstream client and downstream server and they have
the timeout for their own.  Proxy just reacts to their decision.

src/shrpx.cc

@@ -489,10 +489,10 @@ void fill_default_config()
   mod_config()->downstream_write_timeout = {30, 0};
 
   // Read timeout for HTTP/2 stream
-  mod_config()->stream_read_timeout = {30, 0};
+  mod_config()->stream_read_timeout = {0, 0};
 
   // Write timeout for HTTP/2 stream
-  mod_config()->stream_write_timeout = {30, 0};
+  mod_config()->stream_write_timeout = {0, 0};
 
   // Timeout for pooled (idle) connections
   mod_config()->downstream_idle_read_timeout = {60, 0};
@@ -683,11 +683,12 @@ Timeout:
       << get_config()->upstream_write_timeout.tv_sec << R"(
   --stream-read-timeout=<SEC>
                      Specify read timeout for HTTP/2 and SPDY streams.
+                     0 means no timeout.
                      Default: )"
       << get_config()->stream_read_timeout.tv_sec << R"(
   --stream-write-timeout=<SEC>
                      Specify  write   timeout  for  HTTP/2   and  SPDY
-                     streams.
+                     streams.  0 means no timeout.
                      Default: )"
       << get_config()->stream_write_timeout.tv_sec << R"(
   --backend-read-timeout=<SEC>

src/shrpx_downstream.cc

@@ -949,8 +949,13 @@ void Downstream::init_upstream_timer()
 {
   auto evbase = upstream_->get_client_handler()->get_evbase();
 
-  upstream_rtimerev_ = init_timer(evbase, upstream_rtimeoutcb, this);
-  upstream_wtimerev_ = init_timer(evbase, upstream_wtimeoutcb, this);
+  if(get_config()->stream_read_timeout.tv_sec > 0) {
+    upstream_rtimerev_ = init_timer(evbase, upstream_rtimeoutcb, this);
+  }
+
+  if(get_config()->stream_write_timeout.tv_sec > 0) {
+    upstream_wtimerev_ = init_timer(evbase, upstream_wtimeoutcb, this);
+  }
 }
 
 namespace {
@@ -1074,8 +1079,13 @@ void Downstream::init_downstream_timer()
 {
   auto evbase = upstream_->get_client_handler()->get_evbase();
 
-  downstream_rtimerev_ = init_timer(evbase, downstream_rtimeoutcb, this);
-  downstream_wtimerev_ = init_timer(evbase, downstream_wtimeoutcb, this);
+  if(get_config()->stream_read_timeout.tv_sec > 0) {
+    downstream_rtimerev_ = init_timer(evbase, downstream_rtimeoutcb, this);
+  }
+
+  if(get_config()->stream_write_timeout.tv_sec > 0) {
+    downstream_wtimerev_ = init_timer(evbase, downstream_wtimeoutcb, this);
+  }
 }
 
 void Downstream::reset_downstream_rtimer()