json: some renames

We are planning to link json_path_node objects in a tree and attach some
extra information to them so that they could be used to describe a json
document structure. Let's rename it to json_token as it sounds more
appropriate for the purpose.

Also, rename json_path_parser to json_lexer as it isn't a parser,
really, it's rather a tokenizer or lexer. Besides, the new name is
shorter.

Needed for #1012