SSL: reworked posted next events again. Previous change 1ce3f01a4355 incorrectly introduced processing of the ngx_posted_next_events queue at the end of operation, effectively making posted next events a nop, since at the end of an event loop iteration the queue is always empty. Correct approach is to move events to the ngx_posted_events queue at an iteration start, as it was done previously. Further, in some cases the c->read event might be already in the ngx_posted_events queue, and calling ngx_post_event() with the ngx_posted_next_events queue won't do anything. To make sure the event will be correctly placed into the ngx_posted_next_events queue we now check if it is already posted.