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

Just a tiny change. 

`  public static LoggingAdapter GetLogger(IActorContext cell, ILogMessageFormatter logMessageFormatter = null)`
was changed to
`  public static LoggingAdapter GetLogger(IActorContext context, ILogMessageFormatter logMessageFormatter = null)`

At the same time I created private constants for LogLevel strings that's were used and repeated twice in the code.