WL#1721: a binary format for master.info and relay-log.info

Instead of writing positions in human-readable format in master.info (same
applies for relay-log.info) using sprintf("%d"), write the position in binary
format (int4store, int8store). Should make a faster
flush_master_info()/flush_relay_log_info().
When the user wants to see the position from the master.info, have a command in
mysqladmin:
mysqladmin decode-replication-info-file the_file