Replace cJSON by rapidjson

[danielmewes]

https://github.com/miloyip/rapidjson is usually faster than cJSON.

I've already started a branch with a partial port in daniel_rapidjson.

[danielmewes]

Implemented in branch daniel_rapidjson, in CR 2801 by @Tryneus.

[mlucy]

@danielmewes -- it might be worth auditing the rapidjson code to see whether they parse and print doubles in a way that doesn't discard information. I don't think we have a test for that right now.

[danielmewes]

@mlucy Good point. It does round differently from cJSON (this shows up in the geospatial tests). I haven't investigated that yet, but it might indeed discard some information on the way. I'll add a separate test and investigate.

[danielmewes]

Merged into next 9504c0d .

The output of JSON-related commands such as coerce_to('string') and to_json_string() as well as some error messages that include user data will change slightly in format due to this.

Old secondary indexes keep using cJSON for these operations for the time being to maintain compatibility.