Changes to CatalogDB::AddEntry()

1. Make CatalogDB::AddEntry() a public proxy method for a private
   method CatalogDB::_AddEntry().

2. The public method opens a DB connection, and closes the DB after
   use. This is a time-consuming operation, so it is unwise to do so
   when calling AddEntry multiple times.

3. When loading entire catalogs into the DB, the methods should call
   the private method _AddEntry(), which does not open/close a DB
   connection. -- This has not yet been implemented completely in this
   commit.

4. FindCatalog opens/closes a DB connection, so after calling
   FindCatalog, we must re-open the DB connection.

5. Allow ID to be specified even when using the Misc catalog.