add RelationOutput symbol and ExpressionAnalyzer

This commit adds a RelationOutput, ExpressionAnalyzer and a 
RelationOutputResolver.

The ExpressionAnalyzer can be used to convert Expression
(from SQL AST) into a Symbol. Using composition over inheritance makes it
possible to easier handle nested relations and re-use the ExpressionAnalyzer
for different sources.

The DataStatementAnalyzer is now marked as deprecated and shouldn't be used
anymore.
haven't this been changed to be threadsafe (put to the context)?