Rework Union Wrapping

[OlivierCavadenti]

• Rework union construction to work with order by, limit, offset, etc.
• Before this PR, union is done before group by, limit, ... so statements position make syntax errors.
• With this PR, union is done after other statements but, as exposed in group by problems in union query #913 (comment), is not only an order problem.
  I also wrapped all queries in parenthesis to avoid syntax error in PG.
  By the way, reordering components (as intended in the issue) dont work also because the order of bindings is not changed.
• Closes group by problems in union query #913
• Closes union/union all bug wrapped queries  #5047

[OlivierCavadenti]

I change wrap logic (main query is wrapped when union query are wrapped) but I still need to investigate how to generate valid SQL with limit, group by, etc. in unions (not so trivial).