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/Write.cxx

@@ -21,6 +21,13 @@
 
 #include <string.h>
 
+bool
+Client::Write(const void *data, size_t length) noexcept
+{
+	/* if the client is going to be closed, do nothing */
+	return !IsExpired() && FullyBufferedSocket::Write(data, length);
+}
+
 bool
 Client::Write(const char *data) noexcept
 {