Aggressively invoke setAttribute() when manipulating with conversation map in session

Follows up on WELD-1130.

As reported on weld-dev mailing list, the conversation map stored in the session was not re-stored when accessed. This is required on Liberty to determine a "dirty" of an attribute when it comes to attribute replication/storing.

We already have a config option RESET_HTTP_SESSION_ATTR_ON_BEAN_ACCESS which enabled this aggressive way of storing beans into session for the sake of replication (this is only used by Liberty as far as I know). We should implement similar aggressive conversation map storing into session and gate it behind the same config option.

Standard Weld behaviour should stay unchanged by this issue.

PRs should be issues into 2.x, 3.1 and master branches.