BOLT 7: clarify how to encode multiple reply_channel_range messages

[sstone]

When multiple reply_channel_range messages are sent back for a single query_channel_range query, they must cover the requested block range without gaps or overlaps:
first_blocknum must be set to the previous reply_channel_range's first_blocknum plus number_of_blocks.

[t-bast]

Actually there's already a pending PR with the same clarification: #560

[sstone]

Not exactly. #560 specifies that replies must cover the block range requested in a query, without gaps at the beginning or the end, which was implied in the description of channel queries.

This PR adds a constraint for chaining multiple replies to a single query: the next reply_channel_range must start right after the last reply_channel_range that was sent. This is what lnd wants and does, but not eclair or c-lightning for example, which will handle gaps between consecutive replies (but c-lightning will not allow replies to overlap).
Different impls do different things here because the specs are not specific enough, which is what I'm trying to fix: do we allow gaps between consecutive replies ?
This PR says 'no we don't', which imho is the best option as it allows the recipient to validate what it receives.

[t-bast]

Gotcha, that means this PR should supercede #560 because it adds something more restrictive.

[t-bast]

Superceded by #737