HPCC-10298 CSV auto discovery should check the validity and uniqueness of the
discovered field name.

Add code for validate the filed name and check its uniqueness.
    All invalid character change to '_'.

    If fieldname isn't unique then add '_'+<field index>+'_'+<timestamp>
    postfix.

Signed-off-by: Attila Vamos <attila.vamos@gmail.com>