Fixes#5830.
When a channel for a node is announced before the node itself is
announced on the network, it's possible that we have channels for a node
but no features defined yet. This was previously logged as a warning
which spammed the log unnecessarily.
To avoid the channel map needing to be re-grown while we fill the cache
initially, we might as well pre-allocate it with a somewhat sane value
to decrease the number of grow events.
