SSL: reworked posted next events again.

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.