First cut at a filter to buffer/coalesce multiple small buckets into a single large bucket.

This implementation is limited to coalescing a single string of small (< MIN_BUCKET_SIZE)
buckets into one large bucket.  If a brigade contains a string of small buckets followed by a
large bucket followed by another string of small buckets, only the first string will be coalesced.

This implementation works very well with the output generated by mod_autoindex though.
The number of iovecs handled by http_core is reduced to three from over a thousand for my
test case.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@86355 13f79535-47bb-0310-9956-ffa450edef68