Change ADOdb fetch mode to ADODB_FETCH_NUM for Oracle

We were previously using ADODB_FETCH_BOTH, which is less efficient as
each field is returned twice in the recordset. Using ADODB_FETCH_NUM
reduces the memory footprint.

This is a workaround for issue 0015426, allowing MantisBT to function
until ADOdb library is patched to fix the problem's root cause.