Introduce hash join operator as alternative to bound joins

[aschwarte10]

In certain use situations bound joins (or generally any nested loop evaluation) might not be the optimal evaluation algorithm.

Consider the following example:

?city a :City .
?city :inRegion :BW .

There might be a large number of cities in the database, which would cause a large intermediate result set as input to the nested loop join.

If there are factors like latency involved, it might make sense to fetch the result sets of both graph patterns individually, and perform a hash join locally.

This issue is about adding the appropriate operator.

It is yet to be decided how and in which cases to activate it.