Yet another user of global qctx object. Making the method(s) non-static requires pushing the system_keyspace all the way down to size_estimate_virtual_reader and a small update of the cql_test_env
Closes#11738
* github.com:scylladb/scylladb:
system_keyspace: Make get_{local|saved}_tokens non static
size_estimates_virtual_reader: Pass sys_ks argument to get_local_ranges()
cql_test_env: Keep sharded<system_keyspace> reference
size_estimate_virtual_reader: Keep system_keyspace reference
system_keyspace: Pass sys_ks argument to install_virtual_readers()
system_keyspace: Make make() non-static
distributed_loader: Pass sys_ks argument to init_system_keyspace()
system_keyspace: Remove dangling forward declaration