Poco bundles offers JSON-library starting with v1.5.0

[nmandery]

See https://raw.githubusercontent.com/pocoproject/poco/poco-1.6.0-release/CHANGELOG

This might allow dropping rapidjson. On the other side seems debian to be stuck on v1.3 and v1.4 of the poco libraries currently: https://packages.debian.org/search?keywords=poco

So this is more of a change for sometime in the future.