Improve DML support.

Support datatype conversion for text, simple LO and various
datetime values.

Fix ifxCloseConnection() to honour a connection which has
opened a transaction. This also fixes a race, where someone
is trying to call ifx_fdw_close_connection() within a query
against an Informix database, which will lead to unpredictable
situations.
Handle IFX_SERIAL8 types correctly.

setIfxInteger() missed to handle a SERIAL8 datatype value
correctly.