Executor: reimplementing window executor in a pipeline manner.

[wshwsh12]

Feature Request

Is your feature request related to a problem? Please describe:

Now the window executor holds one window partition into memory before calculating. In some cases, it holds the whole table in memory and take up huge memory.

Describe the feature you'd like:

Reimplementing window executor in a pipeline manner to reduce the memory usage.

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy:

[ichn-hu]

/assign @ichn-hu