Logging: rename arg "IActorContext cell" to "IActorContext context"

Reason: Looked strange from the outside, that I have to prrovide a cell when I do not have access to it.

Also create private const for LogLevel strings that's currently repeated twice.
Merge pull request #414 from HCanber/logging

Logging: Rename argument "IActorContext cell" to "IActorContext context"