In the current scenario, the shard receiving the remove node REST api request
performs condional lock depending on whether raft is enabled or not. Since
non-zero shard returns false for `raft_topology_change_enabled()`, the requests
routed to non zero shards are prone to this lock which is unnecessary and
hampers the ability to perform concurrent operations, which is possible for
raft enabled nodes.
This pr modifies the conditional lock logic and orchestrates the remove node
execution logic directly to the shard0, hence the `raft_topology_change_enabled()` is
now checked on the shard0 and execution is performed accordingly.
A test is also added to confirm the new behaviour, where concurrent remove
node operations are now being performed seamlessly.
This pr doesn't fix a critical bug. No need to backport it.
Fixes: scylladb/scylladb#24737
43656371cf