Consider introducing unary GetVertices operator to allow predicate pushdown

[szarnyasg]

This issue is related to #380, but it goes beyond its scope.

Consider the query:

UNWIND [1, 2] AS tagId
MATCH (tag:Tag {tagId: tagId})
RETURN tag

This is currently compiled to a join with two legs:

• Dual -> Unwind -> Join
• GetVertices -> Selection -> Join

For correlated queries, it would make sense to use a 'unary GetVertices' operator, which can take the output of an Unwind operator.

For reference, Neo4j also uses a sort of lazy join operator, Apply, which is an efficient approach.

cc @jmarton