Handle scoping issues in SPARQL

[abrokenjester]

Related to #1405 and #2119

Currently, the SPARQL parser and engine use a check whether something was parsed as a group graph pattern to determine if a variable scope change is needed (which influences the chosen join strategy).

This is a bit of a workaround approach and there are many exceptions. Filter conditions as well as named graph patterns are parsed as group patterns, but do not necessarily mean a scope change. In addition, the entire where clause is by definition a group pattern, but a values clause defined outside the where clause is intended to be scoped to that where-clause. We are continually bumping into other corner cases and adapting the workaround as we go.

To fix the issue more robustly, we should introduce a isScopeChange field in the SPARQL AST tree, which defaults to false. In the case of ASTGraphPatternGroups we can inspect the node parent to determine if the group is part of a named graph or a filter condition before returning true or false. The external values clause will need to be treated as a special case in the TupleExprBuilder (in fact it already is, we need to modify it slightly).

Fix and regression tests incoming.