Add TestConnectionClosureRace

This isn't a test case so much as a reproducer that tries to force a
race between client-side connection reuse and server-initiated
connection closure on various protocols. Since it's fairly slow and
resource-intensive, I have marked it with `@Tag("slow")`. The test can
be run directly from an IDE, or from the command line using the
`slow-tests` profile:

  ./mvnw -Pslow-tests

Author: rschmitt