Fix various Ruby warnings

[zarqman]

This PR fixes a bunch of ruby warnings. There are no changes in behavior. Tested on Ruby 3.3.

gems/nats-pure/lib/nats/io/jetstream.rb:14: warning: gems/nats-pure/lib/nats/io/jetstream.rb:14: warning: loading in progress, circular require considered harmful - gems/nats-pure/lib/nats/io/msg.rb
gems/nats-pure/lib/nats/io/jetstream.rb:15: warning: gems/nats-pure/lib/nats/io/jetstream.rb:15: warning: loading in progress, circular require considered harmful - gems/nats-pure/lib/nats/io/client.rb
gems/nats-pure/lib/nats/io/client.rb:623: warning: assigned but unused variable - msg_size
gems/nats-pure/lib/nats/io/client.rb:934: warning: assigned but unused variable - err
gems/nats-pure/lib/nats/io/client.rb:1181: warning: assigned but unused variable - sid
gems/nats-pure/lib/nats/io/client.rb:1818: warning: assigned but unused variable - opts
gems/nats-pure/lib/nats/io/subscription.rb:75: warning: method redefined; discarding old concurrency_semaphore
gems/nats-pure/lib/nats/io/client.rb:758: warning: method redefined; discarding old connected_server