Rename some classes, add comments, some minor refactoring:
1. Rename EventAllocationType to PartitioningScheme, DestinationPIWrapper to StreamDestination, ThreadsWorkerProcessingItem to ThreadsProcessingItemInstance
2. Use StreamDestination to represents a stream's destination. To add a destination, pass an object of StreamDestination to the stream.
Refactoring:
1. Local single thread mode: use PartitioningScheme enum, StreamDestination when connect PI to stream
2. Move StreamDestination to samoa-api utils