lookup_map_txtfile: used buffered i/o rather than reading RewriteMap txt: files

lookup_map_txtfile: used buffered i/o rather than reading RewriteMap txt: files
one byte at a time.

I was horrified to see a dump of a server with hundreds of worker threads in
read() syscalls,  all trying to read a 149k RewriteMap file simultaneously.
each thread finds one key, inserts it into the cache, then the rest of the
information is discarded.  there are further improvements which could be made but
let's try the simplest first.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@161354 13f79535-47bb-0310-9956-ffa450edef68