Event pipe: fixes for complex protocols.

Event pipe: fixes for complex protocols.

1. In ngx_event_pipe_write_chain_to_temp_file() make sure to fully write
   all shadow buffers up to last_shadow.  With this change recycled buffers
   cannot appear in p->out anymore.  This also fixes segmentation faults
   observed due to ngx_event_pipe_write_chain_to_temp() not freeing any
   raw buffers while still returning NGX_OK.

2. In ngx_event_pipe_write_to_downstream() we now properly check for busy
   size as a size of buffers, not a size of data in these buffers.  This
   fixes situations where all available buffers became busy (including
   segmentation faults due to this).

3. The ngx_event_pipe_free_shadow_raw_buf() function is dropped.  It's
   incorrect and not needed.