multishard_combining_reader: pause *all* EOS'd readers
multishard_combining_reader: pause *all* EOS'd readers

Previously the last shard reader to reach EOS wasn't paused. This is a
mistake and can contribute to causing deadlocks when the number of
concurrently active readers on any shard is limited.

scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla