feature: begin flipping well deployed feature bits to the required bit

[Roasbeef]

This issue tracks our progress on this spec Pr: lightning/bolts#1092

Originally, the PR actually just sort of removed the feature bits and them started to assume that they were compulsory. After a bit of discussion, we decided to instead first flip them to required, see what breaks (if anything), then resume the discussion to reclaim feature bits.

The following feature bits are candidates to flip to required status in all relevant locations:

• var_onion_optin
• gossip_queries
• option_data_loss_protect
• option_static_remotekey

Steps To Completion

• Flip all the bits above to required. This means we'll always advetise them as required in: invoice, our node_announcement, and also the init message.

• Deploy changes within a live node test bed

  • Observe any degenerate behavior or broken connections (does flipping the bits result in not being able to connec to peers any longer?).
  • If so, go back to 1.
  • Otherwise, merge, and profit.