Threads: offloading of temp files writing to thread pools. The ngx_thread_write_chain_to_file() function introduced, which uses ngx_file_t thread_handler, thread_ctx and thread_task fields. The task context structure (ngx_thread_file_ctx_t) is the same for both reading and writing, and can be safely shared as long as operations are serialized. The task->handler field is now always set (and not only when task is allocated), as the same task can be used with different handlers. The thread_write flag is introduced in the ngx_temp_file_t structure to explicitly enable use of ngx_thread_write_chain_to_file() in ngx_write_chain_to_temp_file() when supported by caller. In collaboration with Valentin Bartenev.

--- a/src/core/ngx_file.h
+++ b/src/core/ngx_file.h
@@ -78,6 +78,7 @@ typedef struct {
     unsigned                   log_level:8;
     unsigned                   persistent:1;
     unsigned                   clean:1;
+    unsigned                   thread_write:1;
 } ngx_temp_file_t;