View building is an expensive process that takes a long time to complete.
During the build, it's impact on other work should be minimized, even at
the expense of slightly slowing it down.
Instead, view building is currently performed in the the same scheduling
group (gossip) as other high-priority tasks, in particular raft processing,
which slows it down, making races more likely and increasing the number
of retries that need to be done.
While view building is still initiated in the gossip group (as it's the
result of adding a view, which is a schema change), in this patch the bulk
of the view building work is moved to a low-priority, maintenance scheduling
group (named "streaming" after its main use case).
Additionally, a test is added, where we make sure that the scheduling
group is the one most used when building a view.
Fixes https://github.com/scylladb/scylladb/issues/21232Closesscylladb/scylladb#21326
88ab8db944