Move classes Json_string, and Json_saved_parser_state from
opt_histogram_json.cc to the newly created file sql_json_lib.h
Additionally, make the function json_unescape_to_string public earlier
defined as static in opt_histogram_json.cc, by
adding the declaration in sql_json_lib.h
Needed this change so that, the classes and functions can be reused in
other pieces of the code.
