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 call it `trans` (and it is 'to run a transaction'), but the actual object you pass is a connection (in case of sqlite) or a cursor (in case of pymysql/MySQLdb)