This makes 'routing_channel' the primary object in the system; it can have
one or two 'node_connection's attached, and points to two nodes.
The nodes are freed when no more routing_channel refer to them. The
routing_channel are freed when they contain no more 'node_connection'.
This fixes#1072 which I surmise was caused by a dangling
routing_channel after pruning.
Each node contains a single array of 'routing_channel's, not one for
each direction. The 'routing_channel' itself orders nodes in key
order (conveniently the index is equal to the direction flag we use),
and 'node_connection' with source in the same order.
There are helpers to assist with common questions like "which
'node_connection' leads out of this node?".
There are now two ways to find a channel:
1. Direct scid lookup via rstate->channels map.
2. Node key lookup, followed by channel traversal.
Several FIXMEs are inserted for where we can now do things more optimally.
Fixes: #1072
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
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