It may happen that we do pathfinding while also attempt to change the
graph. In this case changing the database, channel state, graph while
reading from the same sources may create deadlocks. To resolve this we
change locking policy in the graph cache when pathfinding.
This commit, adds a new ForEachNode method to the channel graph cache
that assumes the contents won't be modified. This is generally useful,
and will be used in a later commit to optimize some heavy RPC calls.
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.
