Network tx builder: avoid fetching account from store

[Mirko-von-Leipzig]

Currently, the ntx-builder loads the account and notes from the store whenever it wants to create a transaction.

This is fairly wasteful; and we should be able to keep the account in memory completely by

• loading the account from db on actor startup
• applying the transaction delta to the account when it is committed

[SantiagoPittella]

I implemented two versions:

1. The actor self-applying deltas: This approach works using the actor context to stash the AccountDelta its executed tx produced. On each committed block it checks account_last_tx (a tiny indexed read, but a query anyways) to see if its own tx landed; if so it applies that delta to the in-memory account. On expiry it reloads from the DB. From this approach I like that the coordinator and message passing mechanism is untouched, it's just changes in the actor side. But I don't like that we cannot avoid doing a DB read to ensure that it is our transaction the one that actually landed, and that we are applying the delta in two places: the coordinator to apply into the DB account, and the actor to the in-memory account.
2. The coordinator pushing the account: The builder loop (which is the sole DB writer) already computes the new account in apply_committed_block and then pushes that Account to the actor over a per-account watch channel. The actor just swaps it in. The nice part of this approach is that we don't need any extra DB read. The part that I don't love is adding a new watch channel between the coordinator and each account, and the fact that we are sending the full account through the channel.

I guess that both option are better than the existing. I have a small preference for the first one, despite still having to do a query it is a really simple and indexed so it shouldn't suppose a problem.

I will open the PR for the first one but let me know if you prefer the second (or any other new option)