Take invalid chaintip into account for stale blocks

[Sjors]

After a hardfork, if the new chain makes progress, and then the old chain receives a new block, we currently trigger a stale block alert:

We found 2 blocks at height 609136:

000000000000000000b48bb207faac5ac655c313e41ac909322eaa694f5bc5b1 (0.2 MB, 13:44:09 by BTC.com)
0000000000000000000d356bdc89ff3ef170724a83b6f6f5129c4a959ce8bc08 (0.0 MB, 15:13:48 by unknown pool)

We should avoid this alert if node A sees this new block, but the original block is on a chaintip that node B considers invalid. Or somethign like that.