rjson: add parse_to_map helper function
rjson: add parse_to_map helper function

Existing infrastructure relies on being able to parse a JSON string
straight into a map of strings. In order to make rjson a drop-in
replacement(tm) for libjsoncpp, a similar helper function is provided.

scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla