added a mechanism to get ItemData in the active row for the join operation.

The key function of this patch is getDataOfActiveRow(), which has
been newly added. It will be used when the where condition is evaluated.

getData() is similar function. It returns the ItemData from
the joined ItemGroup. Against this, this function returns
the ItemData in the source tables.
changed the function name.

getJoinedRow() ->
getActiveRow()

This function is a virtual function. One of classes that have
this function is for a table. A joined row for single table that
is one of sources of the join operation (i.e. table->getJoinedRow())
doesn't make sense.