reactor: change shares for default IO class from 1 to 200

The number of shares is supposed to be set in the 1-1000 range,
and user-defined IO classes have shares on the order of hundreds
in practice.

Therefore, 1 share is not a sane default, because the default class
can very easily be starved by accident.
We have seen in practice for example in scylladb/scylladb#13753.

(cherry picked from commit e45cef9)

src/core/reactor.cc

@@ -1667,7 +1667,7 @@ reactor::reap_kernel_completions() {
 
 const io_priority_class& default_priority_class() {
     static thread_local auto shard_default_class = [] {
-        return io_priority_class::register_one("default", 1);
+        return io_priority_class::register_one("default", 200);
     }();
     return shard_default_class;
 }