core: core output filter optimizations.

The core output filter used to determine first if it needed to block before
trying to send its data (including set aside ones), and if so it did call
send_brigade_blocking().

This can be avoided by making send_brigade_nonblocking() send as much data as
possible (nonblocking), and only if data remain check whether they should be
flushed (blocking), according to the same ap_filter_reinstate_brigade()
heuristics but afterward.

This allows both to simplify the code (axe send_brigade_blocking and some
duplicated logic) and optimize sends since send_brigade_nonblocking() is now
given all the buckets so it can make use of scatter/gather (iovec) or NOPUSH
option with the whole picture.

When sendfile is available and/or with fine tuning of FlushMaxThreshold (and
ReadBufferSize) from r1836032, one can now take advantage of modern network
speeds and bandwidth.

This commit also adds some APLOG_TRACE6 messages for outputed bytes (including
at mod_ssl level since splitting happens there when it's active).


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1836237 13f79535-47bb-0310-9956-ffa450edef68