Revert "Client: eliminate SetExpired(), call Close() directly"

This reverts commit 58d7804d66.  It
caused a use-after-free bug when Client::OnSocketError() was called
due to a failed write, e.g. if the output buffer was full.

src/client/Event.cxx

@@ -25,11 +25,11 @@ Client::OnSocketError(std::exception_ptr ep) noexcept
 {
 	FormatError(ep, "error on client %d", num);
 
-	Close();
+	SetExpired();
 }
 
 void
 Client::OnSocketClosed() noexcept
 {
-	Close();
+	SetExpired();
 }