Upstream: improved configuration parser diagnostics.

Made it clear when the selected balancing method does not
support certain parameters of the "server" directive.
Mail: initialize the "signature" field of ngx_mail_session_t.

Currently it isn't used, but it can be suitable to distinguish
objects stored in c->data.
Events: removed broken thread support from posted events.

It's mostly dead code.  And the idea of thread support for this task has
been deprecated.
Events: processing of posted events changed from LIFO to FIFO.

In theory, this can provide a bit better distribution of latencies.

Also it simplifies the code, since ngx_queue_t is now used instead
of custom implementation.