Add persistent server connections

Consider using named pipes to open and maintain open connections to servers rather than constantly invoking mysql -e.

Example can be seen here:
https://bugs.launchpad.net/percona-xtrabackup/+bug/1119451

function open_persistent_mysql_pipe()
{
 local pipe=$(mktemp -u)
 mkfifo -m 600 $pipe
 eval "$1=$pipe"
 (tail -f $pipe) | ${MYSQL} ${MYSQL_ARGS} &
}

function close_persistent_mysql_pipe()
{
 local pipe=$1
 echo "exit" > $pipe
 rm $pipe
}