The asynchronous IO library in glibc starts threads that show up as memory
leaks in valgrind.  This commit attempts to avoid the warnings by flushing
all the asynchronous I/O to the log file before exiting.  This only does
part of the job for glibc since it keeps the threads around for some
undefined idle time before killing them, so in addition this commit adds a
valgrind suppression to stop displaying these warnings in any case.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: William Tu <u9012063@gmai.com>