multishard_combining_reader: shard_reader: use multishard reader's permit
multishard_combining_reader: shard_reader: use multishard reader's permit

Don't create a new permit per shard reader, pass down the multishard
reader's one to be used by each shard reader. They all belong to the
same read, they should use the same permit. Note that despite its name
the shard readers are the local representation of a reader living on a
remote shard and as such they live on the same shard the multishard
combining reader lives on.

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