Add toString method that just shows the string content of the Clob.
Add toString method that just shows the string content of the Clob.

This fixes some tests where JdbcClient is used to display content from
mserver in clob columns.

zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb