JSON-RPC message handling blocks sending own messages

## Describe the bug

Everything per each JSON-RPC connection runs mutually exclusive.

https://github.com/Icinga/icinga2/blob/e50eb52291568056686c65ff836b5593715df515/lib/remote/jsonrpcconnection.cpp#L55-L58

Something, like I/O itself, is async. Something, like queueing I/O, is pretty quick. But message handling itself is the slowest part and blocks the entire strand.

## To Reproduce

https://github.com/Icinga/icinga2/blob/e50eb52291568056686c65ff836b5593715df515/lib/remote/jsonrpcconnection.cpp#L99-L104

## Expected behavior

JsonRpcConnection should have a second strand. The following 33 lines should be a callback posted to the new strand.

https://github.com/Icinga/icinga2/blob/e50eb52291568056686c65ff836b5593715df515/lib/remote/jsonrpcconnection.cpp#L99-L131

As it's a strand AND only one thread posts at a time, this should keep messages handling serial, as the code expects.

But then handling wouldn't block I/O.

## Your Environment

* Version used (`icinga2 --version`): e50eb52291568056686c65ff836b5593715df515

## Additional context

ref/NC/820479

	IoEngine::SpawnCoroutine(m_IoStrand, [this, keepAlive](asio::yield_context yc) { HandleIncomingMessages(yc); });
	IoEngine::SpawnCoroutine(m_IoStrand, [this, keepAlive](asio::yield_context yc) { WriteOutgoingMessages(yc); });
	IoEngine::SpawnCoroutine(m_IoStrand, [this, keepAlive](asio::yield_context yc) { HandleAndWriteHeartbeats(yc); });
	IoEngine::SpawnCoroutine(m_IoStrand, [this, keepAlive](asio::yield_context yc) { CheckLiveness(yc); });

	Dictionary::Ptr message = JsonRpc::DecodeMessage(jsonString);
	if (String method = message->Get("method"); !method.IsEmpty()) {
	rpcMethod = std::move(method);
	}

	MessageHandler(message);

	Dictionary::Ptr message = JsonRpc::DecodeMessage(jsonString);
	if (String method = message->Get("method"); !method.IsEmpty()) {
	rpcMethod = std::move(method);
	}

	MessageHandler(message);

	l_TaskStats.InsertValue(Utility::GetTime(), 1);

	auto total = ch::steady_clock::now() - start;

	Log msg(total >= ch::seconds(5) ? LogWarning : LogDebug, "JsonRpcConnection");
	msg << "Processed JSON-RPC '" << rpcMethod << "' message for identity '" << m_Identity
	<< "' (took total " << toMilliseconds(total) << "ms";

	if (cpuBoundDuration >= ch::seconds(1)) {
	msg << ", waited " << toMilliseconds(cpuBoundDuration) << "ms on semaphore";
	}
	msg << ").";
	} catch (const std::exception& ex) {
	auto total = ch::steady_clock::now() - start;

	Log msg(m_ShuttingDown ? LogDebug : LogWarning, "JsonRpcConnection");
	msg << "Error while processing JSON-RPC '" << rpcMethod << "' message for identity '"
	<< m_Identity << "' (took total " << toMilliseconds(total) << "ms";

	if (cpuBoundDuration >= ch::seconds(1)) {
	msg << ", waited " << toMilliseconds(cpuBoundDuration) << "ms on semaphore";
	}
	msg << "): " << DiagnosticInformation(ex);

	break;
	}

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JSON-RPC message handling blocks sending own messages #10268

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