Counter of nested transactions (or nested invocations of methods of
classes derived from Persistent class). BasicMetaobject implements
implicit scheme of detection transaction boundaries. Transaction is
committed when no more active methods exists for the classes controlled
by this metaobject (or metaobjects derived from Basicmetaobject),
i.e. when nestedTransactionCount is equal to zero. It is possbile
to manualy increment and decrement this count, so explictly
set transaction boundaries (see methods beginNestedTransaction() and
endNestedTransaction() below.