planbuilder: push down ordering through filter

[systay]

Description

When decided what should be pushed under what, we made a mistake.
The thinking was that if we need to filter and order the results, we should filter first so we have less to order. For that reason we didn't push ordering under filtering.

Unfortunately, this means that we forced the sorting to happen on the vtgate side, which means that we have to have the full result set before we can sort it, and that can easily lead to OOMs.

Here we are allowing the ordering to be pushed down under the route, which means that MySQL is producing ordered results. These can be merged together one chunk at a time, thanks to the pre-sorting.

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported
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• Documentation was added or is not required

[vitess-bot]

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