Partitions: Exchange operator.

[ajnavarro]

An Exchange will be a Node in charge to parallelize part of the query tree.
The parallelization level will be provided as Analyzer instantiation property.
The Exchange node will basically execute the same part of the tree behind it with a different partition.

Exchange nodes will call to Partitionable.Partitions() method to get the Partition iterator.

It will send that partition to the nethermost node, that must be a Table.
The number of partitions executed at the same time depends on the parallelization provided to the Analyzer.

Exchange node will return EOF when PartitionIter returns EOF and all the executed node threads return EOF.

[ajnavarro]

closed via #330