Supports pipeline event based scheduling.

[stdpain]

Feature request

Currently our pipeline uses poller mode for scheduling. This leads to some of the following problems.

1. An unintended blocking implementation may result in all queries not being scheduled in a timely manner.
2. poller may not be scheduled in time.
3. The pipeline poller consumes an entire CPU core, resulting in a waste of resources.

Describe the solution you'd like

The main purpose of this PR (#49972) is to remove pollers through the observer notify mechanism, and we will continue this work and make better improvements.

It can save one CPU core in the scenarios such as point query/TPCDS.

In the new implementation, we don't need a poller thread at all, or even a block driver queue. And there is no need to introduce additional locks during scheduling.

• [Feature] support event scheduler (part 1) #54260