Update hyperdb (and multiwriter) for deleted flag and HypercoreProtocol header

[bnewbold]

Rendered: hyperdb, multiwriter

These are some extremely-procrastinated changes to the hyperdb DEP (currently in "Draft" status) to support:

• the deleted flag instead of setting value to null for deletions
• note that a HypercoreProtocol message with protocol string hyperdb should be the first message in every hyperdb feed (as per DEP-0007: Hypercore Header)

Along the way I also:

• updated examples (in both hyperdb and multiwriter DEPs) to be consistent with a HypercoreProtocol header message at entry index 0. Math/typos could probably use a second set of eyes... maybe @yoshuawuyts could take a look?
• talked about the Entry/InflatedEntry distinction in the hyperdb DEP. I think the previous state of referring to a single Entry schema that matched the actual InflatedEntry was confusing.

[bnewbold]

cc: @mafintosh @pfrazee

[yoshuawuyts]

Liking the changes! Left some comments, but this is looking good overall! Nice! 🎉

[pfrazee]

👍 No additional comments from me