Scheduler refactor: move transaction create/commit/rollback/retry up into scheduler?

[rdaum]

Right now it's task.rs that does the logic with this.

With the command interpreter loop there's 2 separate transactions: one to match for parsing the command, and another to execute the command. And with support for $do_command there's a third, and once I fix missing huh handling, that's a fourth! Not a good situation. 3/4 of these transactions all occur within the scheduler, and so could be all subsumed under that one place.

I played with optionally passing an already-existing transaction into the task, but the problem with this is that to do the :huh support the transaction will already have been committed when the task completes with error.

So my proposal is to go through and clean things up potentially so that tasks always begin and end with an open transaction, and it's Scheduler that is responsible for the lifecycle of transactions -- passing ownership of them to the task, and getting them back again at completion.

[rdaum]

Decided to do the opposite and push more down into the task.