The child limit is derived from the upload speed the server records for
us, and the server pushes a user's stats to whoever watches that user —
so a client that does not watch itself never learns its own figure and
would carry the default cap forever. Nicotine+ reads the same number off
the stats the server sends about the logged-in user.
We now watch ourselves as each session starts, and the stats that come
back — the initial reply and every later push — set the figure the limit
is derived from. Client::own_average_speed exposes it.