Add a feature bit to allow nodes to opt-out of channel updates

[Roasbeef]

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This PR adds a new section to the 7th BOLT document detailing a mechanism that nodes can use to opt-out of dynamic channel updates. Doing so is primarily a bandwidth optimization, as the node is able to only receive announcements for new channels and nodes joining the network, rather than announcements for each channel update within the network.

Additionally, we propose the additional ability of a node to signal that they don’t wish to receive any dynamic channel updates using an unused feature bit in the init message.

[rustyrussell]

Main points from discussion:

1. This is a premature optimization.
2. It's not something we want to encourage, as it's antisocial (such a node isn't useful for other nodes).
3. We know we have to revisit syncing in 1.1 anyway ( @roasbeef hates the term "dump"! )

So deferring until 1.1...

[cdecker]

I see the need to optimize bandwidth, however, if we go down this road, I'd like to broaden the bit to suppress all gossip messages, not just the updates. This could be used by slow/low-bandwidth nodes to designate a single node as source for its gossip data (by turning the bit off for that node) and suppress all other nodes.

@rustyrussell this concerns the gossip background noise during operation, not the dump-on-connect, for which we already have bit 3.

[t-bast]

Can we close this PR now that we have GossipTimestampFilters that achieve this (unless I'm missing something)?

[ysangkok]

@roasbeef, is this PR superseded by #641 ?