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