Different log message for new partition in OffsetTracker

Currently when Secor starts consuming a “new” partition it logs a warning that the offset for a given partition changed from -2 to *something*, and I’d say that this particular scenario is expected and shouldn’t generate a warning, but rather an info message that consumption of a new partition has started.

For future reference, -2 means to start from the earliest available offset and -1 means to start from the latest available offset, if I remember correctly.