feat: replace schema with message type

[pfletcherhill]

What is the feature you would like to implement?
Replace schema inside message body with a type field inside message data. The type should be a 16-bit number.

Why is this feature important?
Right now message type is inferred from the format of the message and the schema field. We should be able to quickly and more directly parse a message and know what set to route it to for merging.

Will the protocol spec need to be updated??
Yes, the protocol spec should include the table of supported message types with each version.

How should this feature be built? (optional)
Here is an initial list of message types:

• CastShort
• CastRecast
• CastRemove
• ReactionAdd
• ReactionRemove
• FollowAdd
• FollowRemove
• VerificationEthereumAddress
• VerificationRemove
• SignerAdd
• SignerRemove

And here are potential future message types:

• Metadata
• CastLong
• CastPoll
• CastEvent
• CastRSVP
• GroupInviteAdd
• GroupInviteGrant
• GroupInviteAccept
• VerificationBitcoinAddress
• VerificationTwitter
• VerificationDomain

[pfletcherhill]

Implemented #92