Refactor record tester

Refactor and re-use logic for the following parts:
- Arguments that each `Start()` function takes => they should be moved to (opts) structs
- Reuse the code between different testers (recordtester, vodtester, transcodetester)

I started the refactor here: https://github.com/livepeer/stream-tester/tree/rafal/refactor-recordtester