Add support for DROP VIEW

[agarciamontoro]

This PR adds support for DROP VIEW statements, as in

DROP VIEW [IF EXISTS] [db_name1.]view_name1 [, [db_name2.]view_name2, ...] [RESTRICT] [CASCADE]

As each view in the statement may come from a different database, I have decided to model the plan as follows:

• There is a parent node, DropView, that just contains a list of children of type SingleDropView.
• The nodes SingleDropView implement the Databaser interface so the analyzer can resolve the database in each of them.

The actual removal of the views happen in the RowIter from DropView, as we need to:

1. Lock the ViewRegistry mutex.
2. Check that all views in the children member of DropView can be deleted (i.e., they exist or they do not exist but the user has added the IF EXISTS keyword to the statement)
3. Delete all of the views listed if the previous step raised no error.
4. Release the mutex.

I first started implementing the removal in the SingleDropView nodes, but then it was difficult to check that all views could be deleted and then delete them, all in an atomic operation. The final implementation makes this easy with the new DeleteList from ViewRegistry.

[erizocosmico]

Good job! Just two minor things and LGTM