Calling `AsioTlsStream::async_shutdown()` performs a TLS shutdown which
exchanges messages (that's why it takes a `yield_context`) and thus has the
potential to block the coroutine. Therefore, it should be protected with a
timeout. As `async_shutdown()` doesn't simply take a timeout, this has to be
implemented using a timer. So far, these timers are scattered throughout the
codebase with some places missing them entirely. This commit adds helper
functions to properly shutdown a TLS connection with a single function call.