Use sensible default for flush after clear

[robert-bor]

Currently, it must be stated in BeanCollection whether flushing is turned on or off.

Since flushing only works when Lazy is used, an alternative approach would be to turn it on by default when Lazy is used, with the option to turn if off in BeanCollection.

[robert-bor]

Feedback @sptdevos : make sure to just use a single property for determining whether flush must take place.