loggerd: refactor remote encoder

[deanlee]

• refactor struct RemoteEncoder to class EncoderWriter. and split to encoder_writer.cc/h.
• fix memory leaks.
  resolve: memory leak when loggerd/encoderd get out of sync #28857
  resolve: PC: loggerd memory leak #27512
• fix sync issues & simplify sync strategy.
• unit tests for EoncoderWriter

[deanlee]

the new sync strategy is very simple and straightforward: rotate to the next segment if remote segment changes:

openpilot/system/loggerd/encoder_writer.cc

Lines 3 to 27 in cee606a

	size_t EncoderWriter::write(LoggerState *logger, Message *msg) {
	std::unique_ptr<Message> m(msg);
	capnp::FlatArrayMessageReader cmsg(kj::ArrayPtr<capnp::word>((capnp::word *)m->getData(), m->getSize() / sizeof(capnp::word)));
	auto event = cmsg.getRoot<cereal::Event>();
	auto idx = (event.*(info.get_encode_data_func))().getIdx();
	remote_encoder_segment = idx.getSegmentNum();
	if (current_encoder_segment == -1) {
	current_encoder_segment = remote_encoder_segment;
	LOGD("%s: has encoderd offset %d", info.publish_name, current_encoder_segment);
	}
	size_t written = 0;
	if (current_encoder_segment == remote_encoder_segment) {
	if (!q.empty()) {
	written += flush(logger);
	}
	written += write_encoder_data(logger, event);
	} else {
	// rotate to the next segment to sync with remote encoders.
	ready_to_rotate += std::exchange(marked_ready_to_rotate, true) == false;
	q.push_back(std::move(m));
	}
	return written;
	}

If this strategy is fine, then I think this PR could greatly simplify the synchronization with encoderd and improve the robustness. basically, most of the code is in encoder_writer.cc, excluding unit tests, which is about 100 lines or so. It shouldn't be too hard to review. therefore, I have closed all other small fixes based on the original code.

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