gc_grace_seconds should include UJ (Up/Joining) status period

Partial network connectivity (e.g. and MTU mismatch that blackholes jumbo frames) can cause a node to get stuck in a permanent UJ status (as reflected in nodetool).  It's possible the node can stay in this way for an extended period of time.  Once the isolated node rejoins due to a network repair, it can cause extensive data loss to the healthy nodes.

If the node were completely isolated gc_grace_seconds would prevent the node from joining after the specified period.  Other corner cases besides "DN" should be covered if applicable.

Reference: 

JTAC 2018-0303-0029