Fix to merge code from `_create_table_statement` and `get_schema`

Reorganized a lot of the SQL schema creation code.  Now database table is not dropped / create when `PandasSQLTable` object is instantiated, but instead when `insert` method is called on it.  This allow us to `get_schema` to generate `CREATE TABLE` code without actually creating the table.
(Includes updates from #8083, so will have to be rebased when that is merged)
Actually this is not needed anymore.  Changing back (it was needed because sqlite needs to call execute script anytime a SQL string contains more than one statement.   I was creating a table and then adding the primary key in two statements.  Now its done in one go)