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 '_'.

    Use atom table to store fieldnames for checking.

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

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