Remove `fall_back_to_syn_msg` which is not necessary in newer Scylla versions.
Fix the calculation of `nodes_down` which could count a single node multiple times.
Make shadow round mandatory during bootstrap and replace -- these operations are unsafe to do without checking features first, which are obtained during the shadow round (outside raft-topology mode).
Finally, during node restart, allow the shadow round to be skipped when getting `timeout_error`s from contact points, not only when getting `closed_error`s (during restart it's best-effort anyway, and in general it's impossible to distinguish between a dead node and a partitioned node).
More details in commit messages.
Ref: https://github.com/scylladb/scylladb/issues/15675Closesscylladb/scylladb#15941
* github.com:scylladb/scylladb:
gossiper: do_shadow_round: increment `nodes_down` in case of timeout
gossiper: do_shadow_round: fix `nodes_down` calculation
storage_service: make shadow round mandatory during bootstrap/replace
gossiper: do_shadow_round: remove default value for nodes param
gossiper: do_shadow_round: remove `fall_back_to_syn_msg`
2b11a02b67