net/tls: Wait for data_{source,sink}::close()

Fixes scylladb#799

data_{source,sink}::close() return a future. If it is not ready on
close() return, then the current tls session close() may result in use
after free.

Converting close_after_shutdown() to a coroutine and sequentially
co_awaiting on close() addresses this issue. The waiting is done
sequentially, as this is shutdown path anyway.

src/net/tls.cc

@@ -1604,17 +1604,18 @@ class session : public enable_lw_shared_from_this<session> {
     future<> close_after_shutdown() {
         _eof = true;
         try {
-            (void)_in.close().handle_exception([](std::exception_ptr) {}); // should wake any waiters
+            co_await _in.close(); // should wake any waiters
         } catch (...) {
         }
         try {
-            (void)_out.close().handle_exception([](std::exception_ptr) {});
+            co_await _out.close();
         } catch (...) {
         }
+
         // make sure to wait for handshake attempt to leave semaphores. Must be in same order as
         // handshake aqcuire, because in worst case, we get here while a reader is attempting
         // re-handshake.
-        return with_semaphore(_in_sem, 1, [this] {
+        co_await with_semaphore(_in_sem, 1, [this] {
             return with_semaphore(_out_sem, 1, [] {});
         });
     }