Sometimes gossiper operations invoked from storage_service and other components
run under a non-gossiper scheduling group. If these operations acquire gossiper
locks, priority inversion can occur: higher-priority gossiper tasks may wait
behind lower-priority tasks (e.g. streaming), which can cause gossiper slowness
or even failures.
This patch ensures that gossiper operations requiring locks on gossiper
structures are explicitly executed in the gossiper scheduling group.
To help detect similar issues in the future, a warning is logged whenever
a gossiper lock is acquired under a non-gossiper scheduling group.
Fixesscylladb/scylladb#25907
(cherry picked from commit 6c2a145f6c)
Closesscylladb/scylladb#26068
e88e65426e