open_channel message can use default values

[remyers]

We can reduce the size of the open_channel message by using default values for negotiated values:

[chain_hash:chain_hash]
[32*byte:temporary_channel_id]
[u64:funding_satoshis]
[u64:push_msat]
[u64:dust_limit_satoshis]
[u64:max_htlc_value_in_flight_msat]
[u64:channel_reserve_satoshis]
[u64:htlc_minimum_msat]
[u32:feerate_per_kw]
[u16:to_self_delay]
[u16:max_accepted_htlcs

This can save enough bytes to almost fit the open_channel message into a single 210 byte mesh payload. Other savings may also be possible with the channel_flags and tlvs.

This could be implemented by detecting the open_channel message and stripping the redundant data. The same system could reconstitute these values using defaults on the receiving end.