everywhere: make deferred actions noexcept

Prepare for updating seastar submodule to a change
that requires deferred actions to be noexcept
(and return void).

Test: unit(dev, debug)

Signed-off-by: Benny Halevy <bhalevy@scylladb.com>

db/view/view_update_generator.cc

@@ -39,7 +39,7 @@ future<> view_update_generator::start() {
     thread_attributes attr;
     attr.sched_group = _db.get_streaming_scheduling_group();
     _started = seastar::async(std::move(attr), [this]() mutable {
-        auto drop_sstable_references = defer([&] {
+        auto drop_sstable_references = defer([&] () noexcept {
             // Clear sstable references so sstables_manager::stop() doesn't hang.
             vug_logger.info("leaving {} unstaged sstables unprocessed",
                     _sstables_to_move.size(), _sstables_with_tables.size());