First the bad news: there's an unfixable race in the launcher code.
(By unfixable I mean it would take a lot more effort than this change
to fix it.) The good news is that it should only affect super short
lived applications, such as the one run by the flaky test, so it's
possible to work around it in our test.

The fix also uncovered an issue with the recently added "closeAndWait()"
method; closing the connection would still possibly cause data loss,
so this change waits a while for the connection to finish itself, and
closes the socket if that times out. The existing connection timeout
is reused so that if desired it's possible to control how long to wait.

As part of that I also restored the old behavior that disconnect() would
force a disconnection from the child app; the "wait for data to arrive"
approach is only taken when disposing of the handle.

I tested this by inserting a bunch of sleeps in the test and the socket
handling code in the launcher library; with those I was able to reproduce
the error from the jenkins jobs. With the changes, even with all the
sleeps still in place, all tests pass.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #20462 from vanzin/SPARK-23020.