Pass context information to operators on execution

[luckyasser]

From @sekruse on August 26, 2016 8:35

When an ExecutionOperator gets hold of the OperatorContext it can use that information for several aspects:

• a hash join could use cardinality information to find out which side to index and which side to use for probing
• in-loop operators could modify certain configurations based on the current number of iteration

Thus, such context information should be made available to ExecutionOperators.

Copied from original issue: daqcri/rheem#17