Discussion: drive data emission with proctime/now when there's no new data in EOWC queries

[stdrc]

In emit-on-window-close queries, when there's no new data (hence new watermarks) coming, some data can be pending forever inside the internal states of RW, not being emitted.

Some user want to emit these pending data after a period of time even when no new data come in.

A possible solution can be (just a prelimilary thought, still need discussion):

CREATE SOURCE s (
  ...
  event_time TIMESTAMPTZ
  WATERMARK FOR event_time AS (event_time - interval '5 minutes', now() - interval '10 minutes')
)

So that when there's no new event_time, now() - interval '10 minutes' can be used as watermark expression.