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.