Feature/clean up join messups

[romaninsh]

Merge #72 and #71 first.

Then pull from "master".

Fixing duplicates in the same field

The recent PR into DSQL atk4/dsql#100 has introduced a side effect. atk4/dsql#102 adds a check resulting in exception when the following is called:

$insert->set('foo', 1);
$insert->set('foo', 2);

As it turns out SQL Join would execute set() when 'foo' is joined field:

$m['foo'] = 'one';
$m['foo'] = 'two';

Also if the field is defined as-is then used in a join, it would cause a similar mess-up. This PR fixes that.

Unloading and changing Join

Another problem was visible if you change the field in the "join" field then load a different record. Join wouldn't reset the changed field and would slap that new value into an unrelated record.

Now when the record is unloaded, join-items are cleaned properly.

Change in joined field only.

Previously when you change no field and call save() it would do nothing. If you only change the field that is join() and call save() it would still do nothing.

Now steps are taken to make sure the joined table is updated correctly.