In get_children we get the vector of alive nodes with get_nodes.
Yet, between this and sending rpc to those nodes there might be
a preemption. Currently, the liveness of a node is checked once
again before the rpcs (only with gossiper not in topology - unlike
get_nodes).
Modify get_children, so that it keeps a token_metadata_ptr,
preventing topology from changing between get_nodes and rpcs.
Remove test_get_children as it checked if the get_children method
won't fail if a node is down after get_nodes - which cannot happen
currently.
d4fdeb4839