#4230 feat(host): liveRows hands out a TxWait for a write's transaction (P4)

Stack P, P4 of 4. This takes only the live-follow.ts part of 1feca701c: awaitTxId on LiveSubscription, the TxWait type, and onTxWait on LiveFollowDependencies.

A read followed over Realtime hands its caller a wait that settles once a write's transaction arrives on any of its parts. Once its parts are let go, the caller gets none. A polled read has no wait. The area branches that copy this can take it from here instead.

Tests: the wait settles on the part the transaction reaches, and is withdrawn once the read is let go.