… process, ie. no message in the partition queue

- keep bootTimeOffsets local to consumePartition and renamed it to lastAvailableOffsetAtStartup
- at startup invoke GetOffset() to retrieve the explicit index in all cases ('oldest', 'newest', timestamp)
- factored duplicated code into updateProcessBacklog() and updateMetrics()
- when consumePartition() starts, check if backlog has been processed; fixes all cases of backlog processing stuck when no messages need to be received.

Note: partitionLagMetric was previously set to GetOffset(..., OffsetNewest) - msg.Offset and is now set to GetOffset(..., OffsetNewest) - msg.Offset - 1. Other metrics remained unchanged.