Support Index DB Objects and Management

[tsutomi]

Implement Index objects that derive from IDbObject, and extend the DbObjectType to include "Index" as a type.
Indexes are named and include the description of the columns indexed and the kind of index.
Like any other DB object (eg. tables, cursors, routines, etc.) provide a manager that is used to revolve named indexes.

Implement a special mechanism to resolve indexes based on the table columns.

Indexes will provide a specific search mechanism (as it is now doe "ColumnIndex", but with support for multiple columns), that will return a ITable result (or at least a result object that can be shaped as a table).