txn-wal: Port txns_progress_source off the async operator builder

[antiguru]

Motivation

txns_progress_source was the last operator in txn-wal still built on the async Timely operator builder.
Its sibling txns_progress_frontiers is already synchronous, so this removes the remaining async operator from the crate.

Description

The operator's awaits are real persist I/O (open_writer, data_subscribe, rx.recv), so it ports to a synchronous build_reschedule operator paired with a Tokio task that owns the persist resources and the subscription receiver.
The task forwards DataRemapEntry values over an unbounded channel and wakes the operator through an ArcActivator.
The physical_upper emit decision and the capability downgrades stay on the Timely thread, so the operator still emits only when the physical upper advances and downgrades to logical_upper, on a single chosen worker.
A Finished sentinel lets the operator distinguish a clean subscription close from a task panic.
Shutdown reproduces builder_async's two-phase handshake via button() and local_pressed/all_pressed, matching txns_progress_frontiers.

No behavior change is intended.

Verification

Covered by the existing in-memory persist integration tests that drive txns_progress end-to-end (operator::tests::data_subscribe, as_of_until, subscribe_shard_finalize, subscribe_shard_register_forget).

[aljoscha]

I think this is correct 🤞 it's also much simpler than the other operators you de-asyncified.

[antiguru]

Thanks for the review!