unixctl: Avoid 100% CPU for slowly processed requests with another queued.

unixctl: Avoid 100% CPU for slowly processed requests with another queued.

If another request came in on a particular connection while the previous
request was still being processed, unixctl_server_wait() would wake up the
main loop but unixctl_server_run() wouldn't read the request, resulting in
100% CPU use.

I doubt whether this is a real problem because it's unusual for a client
to attempt to make requests in parallel.  I found it while pursuing a 100%
CPU issue but it turned out not to be a bug (the 100% CPU was caused by
a client making requests as fast as possible).

Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Justin Pettit <jpettit@ovn.org>