core: Add ReadBufferSize, FlushMaxThreshold and FlushMaxPipelined directives.

ReadBufferSize allows to configure the size of read buffers, for now it's
mainly used for file buckets reads (apr_bucket_file_set_buf_size), but it could
be used to replace AP_IOBUFSIZE in multiple places.

FlushMaxThreshold and FlushMaxPipelined allow to configure the hardcoded
THRESHOLD_MAX_BUFFER and MAX_REQUESTS_IN_PIPELINE from "util_filter.c".
The former sets the maximum size above which pending data are forcibly flushed
to the network (blocking eventually), and the latter sets the number of
pipelined/pending responses above which they are flushed regardless of whether
a pipelined request is immediately available (zero disables pipelining).

Larger ReadBufferSize and FlushMaxThreshold can trade memory consumption for
performances with the capacity of today's networks.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1836032 13f79535-47bb-0310-9956-ffa450edef68