otel-cli server improvements

I merged #35 with some outstanding TODOs since it seemed safe enough and most of what's left is cosmetic stuff.

Here are the TODOs from the PR. There are a few more in the code:

- [x] fix up attribute conversion to string
- [ ] get rid of that <nil> on the bottom right corner of `otel-cli server tui`
- [ ] write the connection config out to the json --dir
- [x] add TLS support (esp for testing e.g. otel-init-go)
- [x] figure out why some events are sorting out of order
- [x] move client TLS parameters to helpers so they don't show up in server --help
- [x] implement a cap on the in-memory spans for tui mode