Last few failures on Windows caused by jruby/jruby#9018 were fixed
in jruby/ruby#9019.

Fix sliced string escaping

Fix: rails/rails@9061627#r168784389

Because the `depth` counter is inside `JSON::State` it can't be used
concurrently, and in case of a circular reference the counter may be
left at the max value.

The depth counter should be moved outside `JSON_Generator_State` and
into `struct generate_json_data`, but it's a larger refactor.

In the meantime, `JSON::Coder` calls `State#generate_new` so I changed
that method so that it first copy the state on the stack.

Fix concurrent usage of JSON::Coder#dump