Build progress virtual reader uses Scylla-specific
scylla_views_builds_in_progress table in order to represent
legacy views_builds_in_progress rows. The Scylla-specific table contains
additional cpu_id clustering key part, which is trimmed before returning
it to the user. That may cause duplicated clustering row fragments to be
emitted by the reader, which may cause undefined behaviour in consumers.
The solution is to keep track of previous clustering keys for each
partition and drop fragments that would cause duplication. That way if
any shard is still building a view, its progress will be returned,
and if many shards are still building, the returned value will indicate
the progress of a single arbitrary shard.
Fixes#4524
Tests:
unit(dev) + custom monotonicity checks from <tgrabiec@scylladb.com>
85a3a4b458