Feature Request: Add no_scatter flag to vttestserver

[aparajon]

Feature Description

The no_scatter flag is used today in the vtgate (and also vtcombo) to cause the planner to fail when a scatter query occurs. This flag however does not yet exist in vttestserver, which is a useful construct that is often used for local database testing.

Use Case(s)

We have found in practice that scatter queries can be quite dangerous, as scatter queries at higher levels of traffic can overwhelm a database and break things. One forcing function to prevent scatter queries is to use the no_scatter flag. Doing so at the test level in vttestserver can help consumers catch and prevent scatter queries earlier on in the development lifecycle.