LMail: free all allocations on Close()

Don't wait for the Lua GC to eventually collect the IncomingMail
instance; the GC doesn't know about these allocations and will not
consider them in the decision whether to collect.

This change can save a lot of memory on a busy relay.

debian/changelog

@@ -1,5 +1,6 @@
 cm4all-qrelay (0.30) unstable; urgency=low
 
+  * free memory immediately after email has been processed
   * catch logging errors
 
  --   

src/LMail.cxx

@@ -64,6 +64,7 @@ class IncomingMail : public MutableMail {
 
 	int Close(lua_State *) {
 		auto_close = nullptr;
+		Free();
 		return 0;
 	}
 

src/MutableMail.hxx

@@ -44,6 +44,17 @@ struct MutableMail : QmqpMail {
 	explicit MutableMail(AllocatedArray<std::byte> &&_buffer)
 		:buffer(_buffer) {}
 
+	/**
+	 * Clear this object and free all C++ heap allocations.
+	 */
+	void Free() noexcept {
+		*static_cast<QmqpMail *>(this) = {};
+		buffer = nullptr;
+		sender_buffer.clear();
+		headers.clear();
+		account.clear();
+	}
+
 	bool Parse() {
 		return QmqpMail::Parse({(const std::byte *)buffer.data(), buffer.size()});
 	}