refactor(ntx-builder): replace the account actors with a scheduler

[SantiagoPittella]

Summary

Now the loop spawns a task per account, up to --max-concurrent-txs at a time. The task picks notes, executes, proves, submits, and returns what happened, also this loops does the writting.

The scheduler doesn't hold anything in memory per account except which transactions it has submitted and not yet seen committed.

Changelog

[[entry]]
scope       = "ntx-builder"
impact      = "breaking"
description = "Removed `--idle-timeout` and `--max-account-crashes`."

[[entry]]
scope       = "ntx-builder"
impact      = "added"
description = "Added `--max-concurrent-txs` to bound the number of network transactions computed concurrently."

[[entry]]
scope       = "ntx-builder"
impact      = "changed"
description = "Network transactions are now built by short-lived per-account attempts driven by the committed-block loop."