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

Closes #8097

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)
you can also return the connection in `get_session` with `engine.begin()`, then you don't need to get back the connection from trans (see http://docs.sqlalchemy.org/en/rel_0_9/core/connections.html#using-transactions)