The query namespace is used for symbols which span the coordinator and
replica, or that are mostly coordinator side. The querier is mainly in
this namespace due to its similar name, but this is a mistake which
confuses people. Now that the code was moved to replica/, also fix the
namespace to be namespace replica.
The querier object is a confusing one. Based on its name it should be in
the query/ module and it is already in the query namespace. But this is
actually a completely replica-side logic, implementing the caching of
the readers on the replica. Move it to the replica module to make this
more clear.
