otel-cli server improvements

[tobert]

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