Event pipe: reduced number of file buffers used. If possible we now just extend already present file buffer in p->out chain instead of keeping ngx_buf_t for each buffer we've flushed to disk. This saves about 120 bytes of memory per buffer flushed to disk, and resolves high CPU usage observed in edge cases (due to coalescing these buffers on send).

--- a/src/event/ngx_event_pipe.h
+++ b/src/event/ngx_event_pipe.h
@@ -30,8 +30,6 @@ struct ngx_event_pipe_s {
     ngx_chain_t      **last_in;
 
     ngx_chain_t       *out;
-    ngx_chain_t      **last_out;
-
     ngx_chain_t       *free;
     ngx_chain_t       *busy;