HTTP API: support streaming JSON responses

[julianbrost]

Currently, responses to HTTP requests are fully assembled in memory before they are sent to the client. Given that these responses can get quite big, for example when fetching an unfiltered /v1/objects/services list in a large installation, this can result in (to put it mildly) suboptimal memory usage.

Status quo

1. After an object is determined to be included in the response, it is serialized (this creates a copy of all its attributes):

   icinga2/lib/remote/objectqueryhandler.cpp

   Line 229 in 8d607d2

   result1.emplace_back("attrs", SerializeObjectAttrs(obj, String(), uattrs, false, false));

2. It is then stored in a results vector (which will contain O(response size) memory in total after all serialized objects were appended):

   icinga2/lib/remote/objectqueryhandler.cpp

   Line 313 in 8d607d2

   results.push_back(new Dictionary(std::move(result1)));

3. This is then passed to HttpUtility::SendJsonBody():
   

   icinga2/lib/remote/objectqueryhandler.cpp

   Lines 316 to 321 in 8d607d2

   	Dictionary::Ptr result = new Dictionary({
   	{ "results", new Array(std::move(results)) }
   	});
   	response.result(http::status::ok);
   	HttpUtility::SendJsonBody(response, params, result);

4. Which in turn serializes the whole vector containing all the objects into a big JSON string that's then added to the response object:
   

   icinga2/lib/remote/httputility.cpp

   Lines 60 to 61 in 8d607d2

   	response.body() = JsonEncode(val, params && GetLastParameter(params, "pretty"));
   	response.content_length(response.body().size());

Desired behavior

Stream the JSON to the client's HTTP connection on the fly while going through the objects. This should allow answering these requests using far less memory (probably something like what it takes to represent a single object).

I'll create some sub-issues to break this into smaller tasks to start with.