perf(router) read single Host header

We now avoid reading all request headers via `ngx.req.get_headers()` and
use the `ngx.var` API instead. In the future, we'll distinguish Host
fetching and full headers fetching when arbitrary headers matching will
be added to the router.

kong/core/router.lua

@@ -455,18 +455,18 @@ function _M.new(apis)
   end
 
 
-  local grab_headers = #wildcard_hosts > 0 or next(plain_indexes.hosts) ~= nil
+  local grab_host = #wildcard_hosts > 0 or next(plain_indexes.hosts) ~= nil
 
 
-  local function find_api(method, uri, headers)
+  local function find_api(method, uri, host)
     if type(method) ~= "string" then
       return error("arg #1 method must be a string")
     end
     if type(uri) ~= "string" then
       return error("arg #2 uri must be a string")
     end
-    if type(headers) ~= "table" then
-      return error("arg #3 headers must be a table")
+    if host and type(host) ~= "string" then
+      return error("arg #3 host must be a string")
     end
 
 
@@ -475,7 +475,6 @@ function _M.new(apis)
 
     method = upper(method)
 
-    local host = headers["host"] or headers["Host"]
     if host then
       -- strip port number if given
       local m, err = re_match(host, "^([^:]+)", "jo")
@@ -614,21 +613,18 @@ function _M.new(apis)
     local uri = ngx.var.uri
     local new_uri = uri
     local host_header
-    local headers
+    local req_host
 
 
-    --print("grab headers: ", grab_headers)
+    --print("grab host header: ", grab_host)
 
 
-    if grab_headers then
-      headers = ngx.req.get_headers()
-
-    else
-      headers = empty_t
+    if grab_host then
+      req_host = ngx.var.http_host
     end
 
 
-    local api_t = find_api(method, uri, headers)
+    local api_t = find_api(method, uri, req_host)
     if not api_t then
       return nil
     end
@@ -655,7 +651,7 @@ function _M.new(apis)
 
 
     if api_t.preserve_host then
-      host_header = ngx.var.http_host
+      host_header = req_host or ngx.var.http_host
     end