changed variable name active_record => base_klass

Current code stores the klass name in active_record and this
is used throughout. While reviewing the code time and again
I had the mental picture of active_record being an instance of
a klass. However here the actual klass is being stored in
@active_record.

Secondly at two different places while referring to @active_record
the comment refers to it as base klass. All this points to
active_record being not the best variable name.

So I thought it is better to replace active_record with base_klass.

This change is confined to JoinDependency, JoinBase, JoinPart and
JoinAssociation - all joining related work.