Add a Transaction for unpinned private messages

[awrichar]

Currently, the way to send a private message without pinning a proof to the blockchain is to pass "txtype": "none" when sending the message. This is because Transactions and Operations were originally limited only to those things that resulted in a proof on the blockchain - so neither was created for unpinned messages.

Given the recent split of Transactions (a grouping mechanism) and BlockchainEvents (things that happened on the blockchain), it now makes sense that unpinned messages should be wrapped in a Transaction and Operation for tracking their asynchronous status. Suggest that this new type of transaction be called "unpinned" ("none" can remain as a deprecated synonym).

This also implies that the sending logic for unpinned messages should be further unified with the sending logic for pinned messages, and should generate similar Operations (minus the blockchain-related ones). It further implies that unpinned messages should be batched in the same manner as pinned ones, so that in all cases the Messages can be tied to Batches which are tied to Transactions.

[awrichar]

Given recent developments on #421, it sounds like a redesign of the batching logic may be upcoming. This change should likely follow (or accompany) that redesign.

[awrichar]

#501