This patch implement the TODO [ALTER DATABASE foo OWNER TO bar].

This patch implement the TODO [ALTER DATABASE foo OWNER TO bar].
It was necessary to touch in grammar and create a new node to make home
to the new syntax. The command is also supported in E
CPG. Doc updates are attached too. Only superusers can change the owner
of the database. New owners don't need any aditional
privileges.

Euler Taveira de Oliveira