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/Client.hxx

@@ -122,9 +122,15 @@ public:
 
 	using FullyBufferedSocket::GetEventLoop;
 
-	void Close() noexcept;
+	gcc_pure
+	bool IsExpired() const noexcept {
+		return !FullyBufferedSocket::IsDefined();
+	}
 
-	using FullyBufferedSocket::Write;
+	void Close() noexcept;
+	void SetExpired() noexcept;
+
+	bool Write(const void *data, size_t length) noexcept;
 
 	/**
 	 * Write a null-terminated string.